The starting point for improving our sleep is understanding what constitutes “good” or “normal” sleep. Moreover, there are many commonly held beliefs about what constitutes “good sleep”, some of which are incorrect. We will be discussing what normal sleep looks like to give you a jumping off point for your own sleep health improvement.
Human beings actually did not evolve to “sleep through the night”. Think about it: it would not be advantageous to our survival to be totally unresponsive to the world for a third of the day. Consequently, our sleep evolved to occur in cycles lasting around 90 minutes, with a brief awakening occurring after each sleep cycle (see figure 1, above) and at other times during the night as well. Studies suggest that healthy adults actually wake up an average of 20-43 times during the night. Each 90-minute sleep cycle starts with our brain being awake. Then we progress through the different stages of sleep: Stage 1, Stage 2, Stage 3 (also known as slow wave sleep, which is the deepest stage of sleep) and REM (Rapid Eye Movement sleep, the stage of sleep where we tend to have vivid dreams).
After each sleep cycle, our brain wakes up and does a quick mental inventory of our body and surrounding environment:
- Am I lying on my arm?
- Is my bladder full? Is my back stiff?
- Is my bed partner there?
- Is the sunlight coming through the window?
- Is it too hot or too cold?
If nothing is amiss, our brain may change our body position and put us right back to sleep. Often, we won’t even remember having woken up by the next morning. However, if during these natural pauses in sleep the brain realizes that your bladder is full, or your back is hurting, or the room is too hot, or an intrusive thought has been on your mind and pops into consciousness, the otherwise brief awakening may become more prolonged and something which we do remember in the morning. It’s important to note that just because we remember an awakening does not mean that whatever we did during it was the cause of the awakening. Nor does a remembered awakening necessarily mean that the sleep was inadequate or abnormally fragmented. Awakenings happen during the natural break between sleep cycles and we go right into the next sleep cycle once we fall back asleep.
Our sleep is also naturally less deep and more fragmented in the second half of the night, which might help us prepare for waking. We typically do not go into stage 3 sleep, the deepest stage of sleep, in the 90-minute cycle or two before our habitual wake time. It can be perfectly normal to wake up several times within a sleep cycle as we approach the morning. As it gets closer to our natural wake up time, we are less sleepy (since we have obtained nearly all the sleep we needed that night), so it is more difficult to get back to sleep if we are awakened by something in our environment.
In conclusion, whether we are aware of waking or not, our brain does not actually sleep through the night. Additionally, there are different types of sleep and our sleep is naturally deeper in the first half of the night.
Understanding our Alertness Level
As you’ve likely noticed, our alertness level varies over the course of the day and night. We have reduced alertness right after waking; it improves over the course of the morning; declines a bit after lunch (which is why some people feel the urge to nap after lunch); improves a bit again; and finally declines again as we go to sleep for the night. Interestingly, some people also experience a slight increase in alertness after about 4 hours of sleep. This can result in an awakening for around an hour, after which they are again able to fall back to sleep.
Our level of alertness at any time of day is determined by 2 opposing neurochemical signals in our brains:
- a sleepiness signal (mediated primarily by the energy breakdown product adenosine)
- a wakefulness signal (mediated primarily by the wakefulness neurotransmitter orexin)
As our brain uses energy during the day, the sleepiness signal gets stronger. To counteract this, our brain produces an opposing wakefulness signal. When we are sleep deprived, the sleepiness signal gets stronger, making us less alert and sleepier during the day and night. When we are under stress or feel anxious, the wakefulness signal gets stronger, meaning that we are less sleepy during the day and night.
Understanding these drivers of sleep and wakefulness is helpful because we can manipulate our levels of sleepiness and wakefulness using proven techniques such as sleep compression and stress reduction. These techniques are expanded upon in Knit’s healthy sleep development program, developed specifically for each person based on the most acute areas of improvement identified during the assessment period for the sleep health baseline report.
The exact timing of our sleep and alertness level is dictated by our body’s internal clock, also known as our “circadian rhythm”. Sleep quality is maximized by sleeping during the time your body perceives as night. Keeping a predictable schedule with regular sleep-wake times, regular meal times, getting plenty of bright light in the morning and dim light in the 2 hours before bedtime will help create a strong internal clock, allowing our body to function at its best. When we keep a predictable schedule, our body knows when to expect sleep and starts to prepare itself for sleep even before we get into bed. It also knows when to expect us to wake up and when to expect our meals. It is common for adults to shift their bedtimes and wake times to later on weekends. However, doing so is equivalent to subjecting oneself to jet lag for 2 out of 7 days each week. Staying up late and sleeping in on weekends makes it more difficult to fall asleep and wake up during the work week.
YOU and Normal Sleep
Now that you have a sense of what normal sleep looks like, here are some ways to see how your sleep stacks up (hint: your sleep probably is not as “abnormal” or “bad” as you think!):
- Use the Knit Sleep Assistant App to look at your own sleep stage data. In the context of what you just learned about the normal pattern of sleep, the areas marked “still sleep” can be either slow wave sleep or REM sleep. Do you see evidence of 90-minute sleep cycles? Are there wake-ups recorded between sleep cycles? Do you see an increased number of wake-ups as the night goes on? (Please note: People rarely have cycles that are exactly 90 minutes, so don’t worry if you’re slightly off.)
- Think about your level of alertness over the day and night and relate it to the Alertness diagram from earlier. Do you feel maximally alert immediately upon awakening? Probably not and this is normal. Do you feel a dip in alertness after lunch or a surge in alertness after several hours of sleep?
- Use Trends in the Knit Sleep Assistant App to look at the regularity of your schedule. Are you going to sleep and waking up at the same time on both weekdays and weekends? If not, you need to regularize your schedule to create a robust circadian rhythm.
Curious minds want to know: let the Knit panel of sleep experts know how you compare to normal sleepers. Email us here!