A Night in the Life of a Sleeper

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sleeping.” by John Huss is licensed under CC BY-NC-ND 2.0

 

The rabbit zipped through the underbrush, abruptly changing its direction. It had probably lost its fox pursuer by now, but it couldn’t even spare a moment’s glance. Its instincts were in overdrive – this was about survival. Finally, as the sun dipped under the horizon, it circled around a tree and slipped into a crevice. It wasn’t safe – it was never safe – but it was at least safer than it was before. Its heart gradually slowing, the rabbit stretched out on the ground, and proceeded to lay motionless and unaware for eight hours, completely vulnerable to any passerby.

Wait, what? Well, the rabbit went to sleep.

Sleep is a pretty remarkable thing. It seems obvious to us that we need to rest and recharge once a day for a few hours. But in the world of nature, where every moment is a fight to live another day, spending hours exposed and vulnerable sounds like suicide. And yet, every known animal sleeps in some way or another. That means sleep is more than just a nightly bother – it’s one of the most crucial processes to complex life. And researchers are just beginning to probe its inner mechanisms. To understand sleep, let’s follow a night in the life of a typical sleeper: you.

Our journey begins in the evening, as bedtime approaches. Two main factors battle to decide when you’ll go to sleep: the homeostatic force and circadian rhythm. The homeostatic force is simple – the longer it’s been since you’ve last slept, the more sleepy it will make you. This force is mainly concerned with ensuring you get enough sleep to survive, no matter when or how you sleep. The circadian rhythm, on the other hand, is indifferent to how much you slept last night. Instead, it tries to make sure you sleep at the right time. As the evening drags on, the homeostatic force will start pushing for sleep, but the circadian rhythm won’t have it; the circadian rhythm will actually lead to a peak in alertness about two hours before it’s time to sleep. Eventually the circadian rhythm decides it’s time, and drowsiness sets in. [1,2,3]

At this point, you might disagree with the circadian rhythm and want to stay up later. One neurotransmitter that helps get you sleepy is adenosine, so a popular option is to consume caffeine, a chemical that blocks adenosine. If you stay up too long, though, the brain will think something’s wrong and release cortisol, a stress hormone. This will give you a euphoric feeling; if you’ve ever felt goofy and found everything funny after staying up too late, blame your cortisol.

Finally, you doze off. As you begin to snooze, you enter the first stage of sleep, creatively named “Stage 1”. In this stage, you’re barely asleep at all. Your eyes gently roll behind your eyelids, and you’re still capable of doing many things you can do when awake. If you’ve ever fallen asleep in class, you entered this stage of sleep. In fact, most people who only make it to Stage 1 before waking up again don’t even know they were sleeping at all. You slowly progress to Stage 2, where you truly slip into sleep, and the brain starts showing more sleep-like activity. Eventually, you arrive at Stage 3, the deepest stage of sleep, where your brain waves slow down into long oscillating patterns.

Figure 1: a typical human sleep cycle and its stages. W is for wake, R is for REM (explained below), and N1, N2, and N3 correspond to stage 1, 2, and 3 respectively. These 90-minute cycles repeat throughout the night.

Soon, you enter the final stage of sleep. This stage is called REM sleep, or Rapid Eye Movement sleep, when your eyes dart rapidly behind your eyelids. But it’s better known as the stage in which you dream. During REM sleep, unless you have a sleep disorder, your arms and legs are totally paralyzed to prevent you from acting out your dreams. Even though dreaming is critical to feeling rested, REM sleep is no easy walk through dreamworld. In fact, if you live a relatively sedentary lifestyle, your peak heart rate probably happens while you’re dreaming. And you do dream every night, for almost two hours a night; however, you usually only remember your dreams if you manage to wake up during this REM phase.

You dozed off more than an hour ago, and now you’re peacefully catching some Z’s. Suddenly, you blink awake. You poke your head up, take a look around, maybe check the time. 30 seconds later, you’re back to sleep. This will happen again and again throughout the night. This seems strange – we don’t remember periodically waking up during the night. But in fact, it’s perfectly normal – no one has ever slept for 8 hours straight. Humans wake up several times during the night to check their surroundings and make sure everything’s OK, usually right after finishing a bout of REM sleep. This is a survival feature, something to help counteract the danger sleeping entails.

You return to sleep, making your way across the stages once again; this cycling between stages 1, 2, 3, and REM about every 90 minutes. As the night progresses, you spend more and more time in REM sleep, and the last third of the night is dominated by dreams. Eventually, your homeostatic force is satisfied, and your circadian rhythm agrees that it’s time to get up. Your eyelids aren’t completely opaque – 4-10% of light makes it through to your eyes – so as the sun comes up, special cells in your retina signal that it’s wake up time. Together, all of these factors lead you to wake up. You may hit snooze a few times and catch a couple more minutes of sleep, but they’ll only be Stage 1 sleep, so they won’t leave you any more rested.

So why do we do all of this? Why does the rabbit have to leave itself vulnerable every night? Sleep scientists are still working hard to answer this question. One discovery is that sleep is related to memory and learning. People who sleep after learning a new skill seem to have much better performance after sleeping, for example. Another theory, called “synaptic homeostasis theory,” suggests that sleep is an opportunity for the body to direct more power to the brain to form new neural connections. One piece of evidence for this theory is that the brain uses more energy while it’s asleep. Finally, a new hypothesis suggests that sleep is a time for the brain to clean itself out. During sleep, the circulation of spinal fluid – the fluid responsible for, among other things, carrying waste out of the brain – increases tenfold. Research is ongoing, and the answer is likely a combination of these theories. But one thing is clear: regardless of how vulnerable it makes the rabbit, sleeping is one of the most important things we do.

For more info, take PSYC 135: Sleep & Dreams!

References:

  1. Dijk, D., & Czeisler, C. A. (1994). Paradoxical timing of the circadian rhythm of sleep propensity serves to consolidate sleep and wakefulness in humans. Neuroscience Letters,166(1), 63-68. doi:10.1016/0304-3940(94)90841-9
  2. Lack, L., & Lushington, K. (1996). The rhythms of human sleep propensity and core body temperature. Journal of Sleep Research,5(1), 1-11. doi:10.1046/j.1365-2869.1996.00005.x
  3. Strogatz, S. H., Kronauer, R. E., & Czeisler, C. A. (1987). Circadian pacemaker interferes with sleep onset at specific times each day: Role in insomnia. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology,253(1), R172-R178. doi:10.1152/ajpregu.1987.253.1.R172
  4. Deak, M. C., & Stickgold, R. (2010). Sleep and cognition. Wiley Interdisciplinary Reviews: Cognitive Science,1(4), 491-500. doi:10.1002/wcs.52
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