Stress Part Two: The State of the Science

Welcome back! I hope you enjoyed the introduction to this series in Stress Part One. If you haven’t yet seen Robert Sapolsky’s talk on stress, please do. I’m not kidding when I say he’s one of the best speakers I’ve ever seen – if the length seems ludicrous, just click somewhere in the middle and watch for two minutes, and I’m sure you’ll get something out of it.

I concluded my introduction last time by saying that I’d next delve into the science. As it turns out, I found a whopping 387 studies that passed my inclusion/exclusion filtering, and – as it also turns out – I’m not a very fast reader. Since I need more time to go through them critically, I wanted to tell you a little bit today about what I found and how I found it.

Studies are being published from all over the world

As you can see in the map above (the featured image), the world is paying quite some attention to the impact of meditation and yoga on stress! Each of the icons represents a university or institute that’s published one or more studies regarding the impact of meditation or yoga on stress. Below are close-ups from a few different regions.

Stress studies from the USStress studies from Europe

Stress studies from Asia

Growing attention to the subject over time

Timeline of studies published

I parsed out the 387 studies I’d found by year in order to see the trend over time. I don’t think it’s surprising that the interest in the impact of meditation and yoga on stress has grown exponentially! To give you a different view of the numbers, 286 of the 387 studies were published in the past ten years – i.e. after 2007.

Methods

I wanted to approach this systematically in order to develop a comprehensive view of the available science on the topic, rather than a limited or biased one. I used PubMed to search for the studies; it’s a huge database managed by the US National Library of Medicine and the National Institutes of Health that contains literature from all over the world. On a side note, a lot of articles are free for public access, so if you’re curious check it out!

I searched “meditation OR yoga AND stress” and found 1953 search results. My inclusion criteria were 1) meditation or yoga as an intervention; 2) some component of stress as at least one of the measured outcomes; and 3) availability of the report in English. My exclusion criteria were either a distinct focus in populations with medical/psychiatric illnesses or a focus in niche populations (which had included caregivers of cancer patients, victims of sexual trauma, women who experienced stillbirth, refugees, and inmates.) 391 citations ultimately passed this filtering process, of which 4 were duplicates. I’ve been unable to find or access 45 out of the 387 studies (33 of those 45 were studies published before 2008.) This is all summarized in the diagram below.

Methods Flowchart

Next time I’ll discuss the literature regarding the impact of meditation and yoga on stress for the general population (the 196 studies in the diagram). After that, I’ll discuss the literature on more specific populations such as employees, healthcare personnel, and students (the 146 studies). Thanks for tuning in!

Your friendly neighborhood doctor-in-training/meditation-trainer,

Yogindamind

 

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Stress Part One: How did we get here?

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What even is stress? And how did it possibly seem like a good idea when we were evolving 20,000 years ago??

Robert Sapolsky, a neurobiologist at Stanford University has spent a great deal of time asking and answering these very questions. As it turns out, stress was a wonderful idea when we were evolving into what we are. Now, though? Not so much, and therein lies the trouble.

In his book Why Zebras Don’t Get Ulcers, Sapolsky describes how the stressors of modern day life are completely different than they used to be:

It is a rare event when we have to stalk and personally wrestle down our dinner. Essentially, we humans live well enough and long enough, and are smart enough, to generate all sorts of stressful events purely in our heads. How many hippos worry about whether Social Security is going to last as long as they will, or what they are going to say on a first date? Viewed from the perspective of the evolution of the animal kingdom, sustained psychological stress is a recent invention, mostly limited to humans and other social primates.

(Chapter One, Why Zebras Don’t Get Ulcers, Robert Sapolsky)

So let’s take a quick look at stress: what it is, how it helps, and how it hurts.

What’s Stress?

(Sapolsky himself gives a great overview in this talk, and he’s hilarious! Literally one of the best speakers I’ve ever had the opportunity to see.)

Our body has two major modes of functioning in regard to utilization of resource: the sympathetic (“fight-or-flight”) system and the parasympathetic (“rest-and-digest”) system. When we’re about to perform on stage or ask that person out on a date, our heart beats harder and we sweat a little – our adrenaline is pumping, which means our sympathetic system is roaring. And when we’re chillin’ on the couch, watching TV right after a meal, and maybe snuggled up and just feeling ready to melt – our stress hormones are all pretty low and the parasympathetic system exerts its influence.

We call the sympathetic “fight-or-flight” because it gives a short-term boost to physiologic functioning, and then ideally it ends. We’ve either fought or run away, or our performance is over. Stress, Sapolsky states, is what happens when we start triggering that response because of that meeting, that exam, and the taxes – and the sympathetic system that’s designed for the short-term is kept “on” chronically. Instead of just an adrenaline boost, a long-term response becomes harmful to our body. See, we need frequent and regular intervals of parasympathetic-dominant influence in order to rejuvenate and build up energy stores.

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A simplified view of the parasympathetic system puts it as the rest-and-digest system

So why do we get ulcers while zebras don’t? To paraphrase Sapolsky, we get ulcers because we’re able to activate our stress response with just our mental worries while zebras activate their stress response only during critical situations… like getting attacked by a lion.

My Stress and My Reflux

Let’s take a little trip back to January of my third year of medical school. I was starting my Surgery rotations for the first time, and I’d just developed a cough. It was a rattly, barking kind of cough and seemed to flare at night – I’d wake up in the middle of the night feeling like there was gunk in my windpipes. Fortunately, the cough wouldn’t be present when I would be observing surgery cases in the operating room but mostly just at night and early in the morning. It was also especially worse after eating chocolate. As much as I’d hoped that the gunk would clear and the cough would go away, it just didn’t. Months went on and the cough was still there.

About a month into this problem, I was talking with a friend, one of my classmates, about my cough. “Bro,” he said, “GERD [gastroesophageal reflux disease, or acid reflux] is one of the most common causes of chronic cough. Yeah, listen – I just had a practice question on this.” (Further reading here) As I thought about it, I realized that the pattern of symptoms and correlation with chocolate really did fit with GERD. As it happens, I also had a check-up at the student clinic and an x-ray for one of my applications during this time, which inadvertently ruled out some other things that could explain my cough.

N.B. This is classic med student syndrome – notice a constellation of symptoms and try to diagnose it myself. I did not see a certified health professional or receive a formal diagnosis.

Anyway, I addressed my symptoms by doing two things: 1) stop eating chocolate and 2) relax my body more, especially during and after meals. Once I started paying attention, I noticed that I kept my abdomen really tight all the time. It was the tip of my stress iceberg; I was much more tense and on edge these days than I’d ever been before. I started actively relaxing my body more often and I stopped eating chocolate, and by July, my cough had gone away. Something to think about.

I’m used to being vigilant about my stress levels and how I feel, and normally I cope well with stress through meditation and support. Most of the time, I feel like I’m either minimally or optimally stressed, but third year med school represented the beginning of newer challenges, and it took me some time to identify these issues and adapt to them. Yet, the ability to notice what’s going on within myself and feel confident in trying to address the problems really allowed me to embrace these challenges and overcome these hurdles – just like how I observed how and why my body was unwell and then took the right steps to make it better.

Does my meditation practice help me deal with stress? Absolutely. What does the science have to say, though? Let’s take a look next time at what the scientific community’s been able to discern about meditation and stress so far.

References

  1. Madanick, R. D. (2013). Management of GERD-Related Chronic Cough. Gastroenterology & Hepatology9(5), 311–313.
  2. Sapolsky, Robert M. (2004). Why zebras don’t get ulcers, 3rd ed.. New York: Owl Book/Henry Holt and Co..

Rest

I thought for a second about making the Sleep series a trilogy of five (and it wouldn’t be the first), but then I decided to give it a rest…

music-rest-pun

(pause for applause) … Anyway,  I’ve been thinking a lot about this topic recently. I’d thought about it a lot five years ago as well and pretty much picked up where I had left off.

I realized several years ago that I frequently looked forward to rest. I don’t mean sleep but rather rest from work. Looking forward to the end of the day, looking forward to the end of the week, to the end of the course, to the next break…

It felt like I was constantly running away from the present and I didn’t like the idea of that. Since I started medical school, I kept thinking about the rigor yet to come: the difficulties and workload of a resident/physician threaten to topple one’s balance. Too often, it happens to such an extent that residents and physicians burn out – a term that means loss of enthusiasm for work, feeling cynical, and having a low sense of personal accomplishment. Resident burnout and physician burnout are reaching critical levels in the country. Even the lowest rates of burnout are as high as 40-50%. (A study* was recently completed in York Hospital, York, PA in which researchers evaluated the utility of Heartfulness meditation in reducing burnout among residents and faculty. Preliminary results are very encouraging! Stay tuned.)

In that kind of circumstance, what would happen if I kept thinking about rest? Probably, it would be more painful when I don’t get proper rest, which I’m sure will happen. Probably, I’d be unhappy most of the time that I work (which would be most of the time) and left trying to indulge in short-lived breaks. Probably, I would burn out.

I remember writing in my diary one day that just thinking about rest started to seem really draining, like it was sapping energy I could use for everything else. I was mentally weary of it. Maybe, I wrote, true rest is rest from the thought of rest. Does that make sense? What if I’m so much at peace that even when working hard and feeling tired, I’m not thinking about rest? Rest happens, work happens, but I’d be the same through it all.

My plan when I started medical school: no more “delayed gratification” tactics! No more grind now, relax later! Find the bliss in the now before now is gone and become then. This way, I’d be ready to keep working at all times, and equally ready to rest rightly. Three and a half years later, I think I’m pretty much there. Not that it was easy, but hey, peace and bliss right? Actually, to me it matters more that I can do my work wholeheartedly without having to think about when I’ll stop.

So, I hope, I won’t think about this again except to deepen what I’ve learned. (Time to give it a rest! Ba-dum-pshhh ohwaitdidIusethatonealready..)

 

Sincerely,

YogiNdaMind

 

*Principle Investigator: Dr. Jayaram Thimmapuram, MD, MRCP

 

Sleep Part Three: Sleep, Science, and Meditation

So far, I’ve ranted about how we view and treat sleep in our society and about my personal mess of a journey with sleep. Today, I’ll talk about what the science says about meditation and sleep, and by the end you’ll be an expert on the matter!

But first, many people my age who’re writing blog posts never even bother to give us dates or tell us where they are. This is Friday and I’m back in the library chair …and it’s cloudy outside.

I mentioned in my last post that I stopped using alarm clock some time ago. Third-year of medical school was the biggest challenge yet, and my Surgery rotation was the exemplar. Well, to follow-up, I’m a fourth-year now – and no alarms to date. Not even on a 24 hour call when I had 1.5 hours to sleep before going to morning rounds. It’ll only get harder as I go, but I felt good after that one.

Now on to the main event.

A Review of Meditation for Insomnia

Process: I searched Ovid Medline (the National Library of Medicine’s database for journal articles) as well as PubMed (the NIH database for articles) with a Keyword search: “Meditation” AND “Sleep.” I chose randomized controlled trials (see terminology below) that included a meditation practice as an arm with primary endpoint of sleep parameters or quality. I excluded studies that were conducted in patients with cancer or other medical conditions. I ended up with four studies, which I’ve analyzed and described below with my interpretation to follow.

Terminology: Words have very specific meaning in this realm, so a few definitions at the outset will be very helpful.

  • randomized controlled trial. This is the gold standard of scientific evidence. Two things are directly compared (usually treatment of interest and a suitable control) with other variables kept the same (controlled) and participants randomly assigned to either group. The randomization eliminates the possibility of unseen characteristics of the participants having an effect.
  • significant. Any time I say “significant” in this discussion, I mean statistically significant. In scientific studies, statistical significance is the method to determine if an observed difference is due to chance or not. Almost always, the threshold is defined as 5% – a difference is statistically significant if (through analysis) the probability of it happening purely by chance is 5% or less.
  • mindfulness-based stress reduction (MBSR). MBSR is a relatively specific and standardized program developed using mindfulness meditation. It has been applied in multiple contexts.
  • total wake time. A sleep parameter that is a measure of sleep quality in these studies. It refers to the amount of time awake in bed, either trying to fall asleep or in episodes of wakefulness interrupting sleep.
  • sleep onset latency. Another sleep parameter. The time it takes to fall asleep after getting in bed.
  • et al.. Latin for “and others;” when a study involves more than two authors, we typically say “First Author et al..”

The Studies

In my description of these studies, I intentionally do not include all of the details but only those I consider important. This includes the major results and conclusions, but I encourage any who desire specific details to ask me or refer to the studies themselves.

Additionally, in all of the meditation programs, weekly sessions were held but the participants were also taught and instructed to meditate by themselves at home.

Gross et al. 2011

  • Objective: Investigate potential of MBSR as treatment for insomnia versus pharmacotherapy (PCT)
  • Participants: Adults aged 18-65 years (average around 50) with insomnia and no medical, mental, or sleep disorders related to the insomnia
  • Study Design: 30 adults. 20 randomized to MBSR (eight weekly 2.5 hour classes) and 10 to receive Lunesta.
  • Outcomes: Subjective, sleep diaries and questionnaires.
  • Results: At five months, both MBSR and PCT groups had a significant increase in total sleep time (30 min. increase in MBSR). Only the MBSR group had a significant reduction by 20 minutes in sleep onset latency (see terminology). They performed similar in most other measures.
  • Limitations: The two groups after randomization had one major difference. 9 out of 20 in the MBSR group had a history of depression or anxiety while only 1 out of 10 in the PCT group did. Small sample size especially just 10 people in the PCT group made it difficult to establish other significant differences.

Ong et al. 2014

  • Objective: Assess efficacy of mindfulness meditation with cognitive-behavior therapy as treatment for insomnia
  • Participants: Adults over 21 years (average around 42) with insomnia, no other sleep disorders, no uncontrolled medical condition suspected to interfere with sleep, no uncontrolled psychiatric condition, and no use of sleep medications
  • Design: Three arms. 54 Participants randomized to either MBSR (19 people), mindfulness-based therapy for insomnia (MBTI) with includes cognitive-behavior therapy elements (19 people), or self-monitoring (SM) (16 people)
  • Outcomes: Subjective and objective, sleep diaries and Insomnia Severity Index questionnaire throughout as well as polysomnography and wrist actigraphy performed at baseline, post-treatment, and 6-month follow-up.
  • Results: MBSR and MBTI groups had a significant decrease in total wake time  while SM did not. This difference was observed with sleep diary and wrist actigraphy data but not polysomnography. MBSR and MBTI groups had significant improvements in Insomnia Severity Index scores while SM did not. MBSR and MBTI had no significant differences between each other.
  • Limitations: Small sample size consisting primarily of Caucasian females

Black et al. 2015

  • Objective: To determine efficacy of mindfulness meditation in promoting sleepy quality in older adults with moderate sleep disturbances
  • Participants: Adults 55 years and older with active sleep disturbance with no other sleep disorders, major medical conditions, or current meditation practices
  • Design: 49 randomized 1:1 into mindful awareness practices (MAPs) group or sleep hygiene education (SHE) group. Both were equal in time commitment.
  • Outcomes: Subjective, primarily the Pittsburgh Sleep Quality Index (PSQI) with other secondary questionnaires.
  • Results: The MAPs group had a significant improvement in sleep quality as well as significant improvement in daytime fatigue and impairment compared to the SHE group. The SHE group did not have any significant improvement in sleep quality, although it did have significant improvement in daytime impairment according to a few measures (compared to its own baseline before treatment).
  • Limitations: No objective measures (noted by authors as due to funding constraints) or use of sleep diaries, so we don’t know how much the sleep improved in the MAPs group (i.e. how much more they were sleeping or less they were awake in bed).

Zhang et al. 2015

  • Objective: To assess effectiveness of MBSR for chronic insomnia in older adults with depression and anxiety symptoms
  • Participants: Adults aged 75 years or older with insomnia and no mental disorders, serious physical illnesses, dementia, or previous training in meditation
    Design: 60 randomized 1:1 into MBSR (eight-week program) or wait-list control group (no formal intervention).
  • Outcomes: Subjective, the Pittsburgh Quality Sleep Index (PSQI), the Self-rating Anxiety Scale (SAS), and the Geriatric Depression Scale (GDS)
    Results: MBSR had a significant decrease in the PSQI global score while the control group did not
  • Limitations: Poor control – the control group didn’t receive any intervention. No objective measures or sleep diaries to estimate effect size.

So what does it mean?

Simply put, the data shows that meditation can help with sleep trouble, at least as much as sleep medications. All of the participants in these studies were new to meditation. Currently, most studies involving meditation use some form of mindfulness meditation.

But here’s what’s interesting. Meditation didn’t exactly help people sleep more! Instead, it helped people fall asleep faster and be awake less during the dedicated sleep period. This even fits my experience as well as anecdotes from friends and family. Practicing meditation seems to help the most during that time that we get in bed and try to fall asleep. Has your mind ever kept you up with thoughts all over the place right? Maybe it’s time to regulate that #monkeymind.

Sincerely,

YogInDaMind

 

References

Black DS, O’Reilly GA, Olmstead R, Breen EC,Irwin MR. (2015). Mindfulness meditation and improvement in sleep quality and daytime impairment among older adults with sleep disturbances: A randomized clinical trial. JAMA Internal Medicine, 175(4), 494-501. doi:10.1001/jamainternmed.2014.8081
Gross, C. R., Kreitzer, M. J., Reilly-Spong, M., Wall, M., Winbush, N. Y., Patterson, R., . . . Cramer-Bornemann, M. (2011). Mindfulness-based stress reduction versus pharmacotherapy for chronic primary insomnia: A randomized controlled clinical trial. EXPLORE: The Journal of Science and Healing, 7(2), 76-87. doi:http://dx.doi.org/10.1016/j.explore.2010.12.003
Ong, J. C., Manber, R., Segal, Z., Xia, Y., Shapiro, S., & Wyatt, J. K. (2014). A randomized controlled trial of mindfulness meditation for chronic insomnia. Sleep, 37(9), 1553-1563. doi:10.5665/sleep.4010
Zhang, J., Liu, X., Xie, X., Zhao, D., Shan, M., Zhang, X., . . . Cui, H. (2015). Mindfulness-based stress reduction for chronic insomnia in adults older than 75 years: A randomized, controlled, single-blind clinical trial. EXPLORE: The Journal of Science and Healing, 11(3), 180-185. doi:http://dx.doi.org/10.1016/j.explore.2015.02.005

 

Sleep Part Two: My Journey

Anyone who knew before medical school can attest that I used to be terrible about sleep. Always late in the mornings and constantly falling asleep in class, I reached a debilitating low before I decided to change and undergo sleep reform.

It all started in fourth grade . . .

One morning we had to be at school by 5:30am in order to drive down to Jamestown for our class trip. I asked my sister to use her alarm clock so that I might wake up and arrive on time. (As a child, I was always fashionably late to school..). That day was the first time I used an alarm clock, and over time it became a symbol of my sleep problems.

By senior year of high school, I was using three alarms every morning: each just an explosion of static at max volume since anything short of it would fail. Thus, waking up became an intensely sympathetic fight-or-flight triggering process, and the struggle of waking every morning was perhaps only matched by the struggle of falling asleep each night.

I was at my lowest in my first semester of undergraduate college. Weekdays I slept from 4am-8am and then 2pm-6pm, and weekends I’d sleep from 6am to 6pm. It started taking it’s toll, even beyond just falling asleep in every class. I starting experiencing hallucinations while waking, termed hypopompic hallucinations, and sleep paralysis so that I would feel awake but couldn’t move my body. Sometimes when falling asleep, it would feel like I took a wrong turn and couldn’t fall asleep properly – I’d lay there in drowsy confusion with my head increasingly pounding as I’d almost fall asleep. I realized I had to do something. My sleep, and as a result my life, was becoming totally unstructured.

In the next semester, I sleep regularly from ~1am-9am; I also went to the gym every single day hoping that I’d feel tired by the end of the day and fall asleep quickly. Unfortunately, that often didn’t work, as my #monkeymind would not cooperate with this brilliant plan.

Over the summer though, I started Heartfulness meditation, and that changed the game. First, it became a tool that helped me change myself as I wanted. It empowered me to transform. More importantly for sleep, it helped regulate that #monkeymind. That silence and stillness at the end of the day became a tranquil time filled with love and a connection to my deeper self, rather than a draining and frustrating forbearance.

Then I imagined, and then I dared. On February 6, 2012, I stopped using alarm clocks. Not even one, ever. Not even when I was up studying ’til 3am for my 8am final. Not even when I was up ’til 4am and had a 6am online meeting to catch. Not even when I slept in four hour chunks when I had ED (Emergency Department) day lectures, night shift, and day shift over 48 hours earlier in my third year (the first year of clinical rotations in medical school.)

In about two weeks, I’ll have been alarm-free for four years. But I’m facing my biggest challenge yet: I’m on my Surgery rotation. Infamous for destroying the circadian rhythm, yet filled with unparalleled excitement for me as I hope to enter a surgical specialty. I’ll let you know how it goes. Here’s hoping.

Meditation seems to have unique benefits for sleep, including perhaps allowing one to rest in fewer hours! Next time, I’ll take about what the science has to say on the matter.

YogInDaMind

 

Sleep Part One – The Situation

This won’t be the first time that I write about sleep, but this is my first blog post(!) Below is one of the first times I wrote formally on my opinion of sleep and our relationship with it, though it is unpublished. Reading it over again, I found not only that I had some decent eloquence at the time but that I had even written what sounds like a great intro to a blog. And so I’ll use a number of my own words this second time around:

The first step is to abandon the consideration of the third person. This is an exchange between me, the writer, and you, the reader. So it is done, and this exchange can now become intimate.

I went to a meditation conference last weekend. On Saturday afternoon, there was a discussion activity organized for those between ages 14 to 25. Subject of discussion? Sleep. Lo! And the beast awakens. Few things I get so carried away about. It’s not that I’m just passionate about it, but sleep – I’m almost angry when I start talking about sleep.

WritingonSleep

One of my writings. Title: Sleep; Date: Feb. 09, 2014

Sleep isn’t given the place it deserves in our society. It may seem an odd statement; after all, we love sleep. If we could, we’d all sleep many more hours of the day. And yet, it’s often the first thing to go, and just like Music and Arts in elementary schools, I find it tragic when that happens. It ranges on the spectrum of the individual to the societal, and we each often feel the brunt of both. On the individual level, maybe we’re hanging out with friends or surfing the net or reading blogs and then pretty soon its 3 am. Or 5 am. It happens – what’s the big deal? We hardly know. On top of our habits, we sometimes don’t have a choice. Night shifts and graveyard shifts, work starting at 6 am, even classes and schools man…

This bit’s not just rant and opinion though! Check it out: the Center for Disease Control (CDC) stated in a report that schools in the United States should start later in the morning. The motivation for the statement comes primarily from the observation that students are sleep deprived. One study found in a representative sample that fewer than one third of high school students sleep at least 8 hours a day (1). To help the kids get more sleep, the CDC concluded, schools should start no earlier than 8:30 a.m.. 82% of schools start earlier than 8:30. The CDC is recommending 82% of US schools to start later in the morning just so kids can get enough sleep.

Sleep deprivation causes tiredness, fatigue, lack of concentration… and increases in BMI? Well, we’re not sure yet, but we’ve found correlation. For adolescents studied from 1994 to 2009, it seems that later bedtimes correlated with increases in BMI of 0.035 kg/m2 per min later bedtime over 6 years. That’s a 2.1 kg/m2 increase every 6 years per hour later.

So sleep is clearly a big deal, but it’s difficult for us for a number of reasons. What if we have 8 hours a night set aside just for sleep, would that 8 hours translate into 8 hours of sleep? After all, we won’t be satisfied with our quantity of sleep if our quality suffers. Ever laid down at night and started getting bombarded by thoughts to the extent of feeling tired while just laying down? Sleepless nights filled with thoughts are painful. Our mind, after running amok all day, is given an opportunity to sleep, but poor thing, it doesn’t know how! A whole other issue presents as an obstacle for us!  Well, as it is unable to voluntarily enter that blissful state of sleep, we are seemingly left with no other choice but to force it, drugged or beaten through a myriad of sensory assaults. I had mainly used music: a few playlists and genres that would run through the night so that I could fall asleep to something. Once I had a laptop, I could now fall asleep watching TV! Perfect, no time for thoughts, no torment. (This TEDx talk on Transforming Solitude really hits the nail on the head here.) Sleep to Netflix, sleep to movies..

What’s the other side of the coin? Kicked down with desperation the night before, we’ve now to make it functional for the coming day. It’s a bit of its own science – so many fluid ounces every few hours or just one bottle every five.

The #monkeymind is not to be taken lightly. I’ve been there, at a time when my relationship with sleep was a very polar love-hate. I went through a period of serious sleep reform after reaching quite the desperate state. I hope it will make for an interesting story, next time.

Sincerely,

YogindaMind

 

  1. Asarnow LD, McGlinchey E, Harvey AG. Evidence for a possible link between bedtime and change in body mass index. SLEEP 2015;38(10):1523–1527.