Monday, September 12, 2011

The Sleep Cycle: What's really going on while you're catching your zzz's

Today I continue on my new quest to understand sleep and its role in our lives by attempting to answer a very basic question: what happens when our heads hit the pillow at night?

The 90 minute Sleep Cycle

The sleep cycle: A sleep cycle lasts about 90 minutes and during that time we move through five stages of sleep. The first four stages make up our non-rapid eye movement (NREM) sleep, and the fifth stage is when rapid eye movement (REM) sleep occurs.

NREM sleep: Across these four stages we move from very light sleep during Stage 1 down to very deep sleep in Stage 4. It is very difficult to wake someone who is in Stage 4 sleep. Across NREM sleep, we have little muscle activity and our eyes do not typically move, but all of our muscles retain their ability to function. 

REM sleep: As the name would imply – during this final stage of sleep, we have bursts of rapid eye movements. This is the stage of sleep in which most dreaming occurs. Our eyes are not constantly moving, but they do dart back and forth, up and down. These eye movements may be related to visual images of dreams, but that is not confirmed, and in general, the reason for these eye movements is still a mystery. Although our eyes are moving rapidly, the muscles that move our bodies are paralyzed (other important muscles, such as our heart and diaphragm continue to function normally). 

So what happens over the course of a night of typical sleep?
It turns out it is not as simple as putting together 4 to 6 of the 90 minute sleep cycles I described above. Over the course of the night, the amount of time we spend in a particular stage of sleep begins to shift. As shown in the figure below, during the first 2-3 sleep cycles, we spend most of our time in deep NREM sleep (stages 3-4), whereas during the final 2-3 sleep cycles, we spend much more time in REM sleep accompanied by lighter NREM sleep. And the complexity of sleep doesn’t end there – apparently how much NREM and REM sleep we get is not just based on where we are in our nightly sleep, but it also depends on what time of day (or night) it is. Regardless of when you fall asleep, people tend to experience more NREM sleep in the earlier hours of the night (e.g., 11p – 3a) and more REM sleep in the later hours of the night (e.g., 3a – 7 a). So those after-hours mutants are getting more REM sleep overall than are the early-to-bedders. As with many other aspects of sleep, the need for all this complexity in our sleep cycles is still a mystery.

5 Sleep Cycles over a typical night of sleep

Changes in sleep across the lifespan: The typical adult sleeps about 8 hours per night (with lots of individual variability – I personally think I need about 9 to 9.5 hours, while my dad can’t seem to sleep more than 6 or 7!). Teenagers tend to need 9 or more hours of sleep a night to be optimally alert the next day (perhaps I’m stuck in adolescence), and sleep needs continue to increase as you move backwards towards infancy. But again, its not as simple as how much sleep we need at various times in our lives, our age also helps determine what type of sleep we get. As we move from childhood to adulthood, we experience a reduction in how much deep sleep we get (NREM Stages 3-4). This change takes place primarily in adolescence when about 40% of NREM Stages 3-4 sleep is replaced by Stage 2 sleep. In addition to losing our deep sleep, we also cut back on REM sleep as we age. Newborns spend half their total sleep time in REM sleep, but by two years old that is down to only a quarter (and remains that way). Babies also sleep on shorter cycles – only 50 – 60 minutes, and can fall asleep straight into REM sleep. 

What does all of this mean for napping? Giving yourself a full sleep cycle (90 minutes) can help you retain certain skill you’ve just learned, but for recovering from fatigue a 15-20 minute nap is ideal (and some research suggests a nap as short as 5 minutes could be beneficial!), since the farther along you are in your sleep cycle, the harder it is to get over that grogginess you sometimes feel when you first wake up (known in the sleep world as sleep inertia). 

Executive Sleep Pod
What's the ideal sleep schedule? Napping leads me to a final comment… although in Western Society we tend to sleep in one big chunk at night, some researchers question whether this is what our bodies were designed to do. Some remote cultures are known to sleep in two phases: about six hours at night and a one and a half hour nap in the afternoon. This sleep schedule is similar to those Mediterranean cultures known for their mid-afternoon “siestas.” This type of sleep schedule may fit better with our circadian rhythms which tend to experience a drop around 2pm (you know you’ve all experienced that post-lunch low!). It seems that some corporations are beginning to recognize the benefits of an afternoon nap, and businesses have even cropped up in large cities to capitalize on this new power-nap craze. 
Would you prefer to be on a biphasic sleep schedule (6 hours at night, 90 minutes in the afternoon)? Do you often nap? For how long? 

Articles and Further reading:
Feinberg I, & Floyd TC (1979). Systematic trends across the night in human sleep cycles. Psychophysiology, 16 (3), 283-91 PMID: 220659
Kohyama J (1998). Sleep as a window on the developing brain. Current problems in pediatrics, 28 (3), 69-92 PMID: 9571325
Takahashi M (2003). The role of prescribed napping in sleep medicine. Sleep medicine reviews, 7 (3), 227-35 PMID: 12927122

NIH teaches about sleep


  1. Very interesting. I would guess that most of us do not track our sleeping patterns in relation to our overall state of mind and physical energy levels. I personally prefer to be awake for long periods and then sleep only when I can't function any longer. Sleep always makes me feel like I missed something and I have to start over. Weird.

    However, I have noticed the older I get (41) the less sleep I need. On my work/school days, I go from 4 AM until 10PM with really no breaks. On these days I tend to take a break (10-15 minutes) to stretch and "meditate" which is actually more accurately defined as a quick nap. I usually feel recharged after this little break and if done between work and school (around 2PM) I'm fully prepared for 3-6 hours of classroom demands.

    Another thought. At several points in my life I've had to change my sleeping patterns based on my activities. Example: When I'm backpacking, I'm up early, physically active, and ready for deep sleep early in the evening. When I was a bartender, I would work for 4 days straight with only a few hours of sleep a night, followed by 3 days off with 10+ hours of sleep per day. Now, my week is broken up even more chaotically. So, I've researched techniques to assist sleep (have you ever needed to sleep, but couldn't fall asleep and you end up in bed, watching your precious hours tick away, knowing you have to be up soon?). Anyone else deal with the conflict between life and sleep? What helps?

  2. For many years I have puzzled over this: when I am unusually tired, I will sometmes fall asleep literally as soon as my head hits the pillow. I will wake up, thinking several hours have passed, and find it's only been 5 minutes.
    Conversely, there are times I think I've only slept a few minutes and several hours have passed.
    Normally my sense of time while sleeping is in sync with 'real world' time.
    Looking at your diagram above, I wonder if, in the first case, I am falling directly into very deep sleep. In the second case, perhaps I'm mostly in light sleep, with occasional drops into deeper sleep.
    The underlying question is, do different sleep stages affect our sense of time? If our brain normally processes time asleep as it does when awake, what causes the brain to lose, or to skew that sense of time?
    And, if we do lose our sense of time in very deep sleep, for example, how then does our brain compensate for that loss such that our overall sense of time while asleep correlates with actual time?

  3. DKM - According to the NIH website, the idea that people need less sleep as they get older is a common misconception. Our sleep needs do decrease from infancy to adulthood, but apparently not across adulthood. However, as you've experienced, older adults tend to actually get less sleep. I guess it becomes harder to deep, restful periods of sleep and thus they are more sensitive to things like light, noise and pain while they are asleep.

    As for the life-sleep conflict, I'm sure we've all experienced it! As for techniques to help sleep - I've heard you should keep electronics (especially tvs and computers) out of the bedroom, and not watch the clock if you can't sleep, but instead get up and do something relaxing. However, I'm not sure what has been studied scientifically, so if someone knows some scientifically-backed methods, I'd love to hear them!

    Margaret - I think what you are saying makes sense in terms of having a better sense of time when you are in a light sleep. I couldn't find any research describing how people perceive time during sleep (though it seems like such an obvious and important question that I feel like there must be research out there on the topic!). I also think there must individual differences with this - some people can consistently wake up at the right time without an alarm, while others of us will never accomplish that feat.

    Thanks for reading!

  4. As soon as I go on vacation I slip into the afternoon nap habit. I also get up early, and go to bed late, so I naturally get into the 6 + 1.5 pattern.

  5. My work schedule is a late swing shift (4:30p-2:30a), and it is perfect for me. When I'm not working I'll naturally wake up around 1:00pm and not feel tired until 5:30am.

    If I need to accommodate day-walker hours, I find that waking up ~7am is manageable, so long as there is coffee after the alarm clock, and a nap before dinner. Weirdly, I will still be awake until at least 1am even if I know I must be up again at 7am. It's not sustainable for more than a couple weeks, but it mostly works. Given the chance, I revert back to the 5:30am-1pm sleep schedule.

  6. @DKM I also feel like sleeping means I'm missing out on something. (Reading, enjoying sunlight, walking, studying...) I very much enjoy sleep once I'm in bed, but often choose to keep going until sleep is unavoidable (too tired to safely function).

    I wonder how common this perception/attitude towards sleep is??

  7. Shirah - there are individual differences in people's sleep needs, you may be able to function on less sleep. Also, as Juli mentioned in her post on the after-hours mutants, some people are naturally early to bed-early to rise, whereas other people are naturally night owls. Perhaps then, you are one of those natural night owls!

    As for avoiding sleep - I often find myself staying up watching tv or reading later than I know I should because I just don't want it to end. Then no matter how much I regret it the next day, I find myself doing the same the next night! I don't know if anyone has systematically examined this phenomenon, but you and DKM are certainly not alone in terms of not staying awake to avoid missing out on life!

    Thanks for your comments,

  8. @Shirah, exactly! There is so much to do during the day that nighttime makes me want to enjoy things I normally have to sacrifice during waking hours. Imagine how many books you could read for pleasure if you didn't need to sleep 6-8 hours a night.

  9. If REM stage of the sleep cycle is where we dream, I wonder why some people would wake up from it.

  10. i have a question Ami this guy connor told me every 15 minutes (starting on the EXACT minute you fall asleep) people fall into rem sleep and if you wake up on rem sleep you experience grogginess and tiredness. He told me this because sometimes when i sleep i wake up really tired . Can you tell me if this true.

  11. Very informative article by the way.
    Thanx :)

  12. seriously speaking i get to know first time about it. it really feel me amazed. how we sleep and about the stages of sleeping
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  13. I am in a Psychology class and we are currently learning about sleep and the different stages that you go in throughout the night. Not until modern technology could we actually see what was going inside someones head while they sleep, but with the creation of the Electroencephalograph, made by a German psychiatrist, Hans Berger, it shined a light on the inside of the human brain by measuring electric activity of rhythmical patterns called brain waves. This device makes an EEG or electroencephalogram. By looking at these EEG's researchers can see how brain waves change throughout the sleep cycle. With each stage of sleep there is a different brain wave pattern. When you are awake and alert you have Beta brain waves, when you are awake and drowsy, right before your Stage 1 NERM sleep, you have Alpha brain waves. Once you are asleep, Stage 1 NERM you have a mix of Alpha and Theta brain waves. Stage 2 sleep you have theta brain waves and the beginning of Delta waves. Stage 3 and 4 NERM sleep or deep sleep you have Delta waves which are very slow waves and your heart rate breathing and blood pressure are at the lowest possible levels. The span of NERM sleep from Stage 1 through Stage 4 takes between 50 and 70 minutes after that you go into REM sleep or rapid-eye-movement sleep where your brain waves are much faster and much more active, it is also when you eyes will rapidly move side to side or up and down hence the name.
    If you nap then you should never get into Stage 3 or 4 sleep and then wake up because it will actually make you more tired because your brain is going from slow brain waves to fast brain waves and it will make you feel more tired and fatigued. Its best to take a nap that lasts 90 minutes or one that lasts about an hour.