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Understanding Normal Sleep and Alertness

By Michelle Jonelis

Obtaining proper sleep at night is essential to functioning well during the day. Unfortunately, there are many misconceptions about what constitutes "normal" sleep and gaining a better understanding of the structure of sleep and the factors which influence daytime alertness can help dispel some of these misconceptions.

What is Normal Sleep?

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 (reference:

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, Stage 4 (stages 3 and 4 are 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 baby breathing?

  • Is the sunlight coming through the window?

  • Is it too hot or too cold?

  • What time is it?

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, or you glance at the clock or your smartphone, 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 the previous night 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 as we approach our typical wake up time. Sleeping less deeply at the end of the night is helpful in preparing us for waking, giving us some awareness of what is happening around us before we rise for the day. We typically do not go into slow wave 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.

How does our Alertness Level Vary Over the Day and Night?

Just as it is helpful to understand that our sleep depth is variable over the course of the night, it is helpful to understand that our alertness level varies over the course of the day and night. When research subjects complete a reaction time test at various intervals over the day and night, a predictable performance pattern emerges. 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 in the late afternoon and early evening; and finally declines as we prepare to go to sleep for the night.

Our level of alertness at any time of day is determined by 2 opposing neurochemical signals in our brains:

  1. a sleepiness signal (mediated primarily by the energy breakdown product adenosine.)

  2. a wakefulness signal (mediated primarily by the wakefulness neurotransmitter orexin, but other neurotransmitters such as dopamine are also involved.)

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.

In conclusion, our level of alertness varies over the day and is influenced by how well rested we are and whether we are under stress.

What is a Circadian Rhythm?

The exact timing of our sleep and alertness level is dictated by our body’s internal clock, also known as our “circadian rhythm” (“circa” means around, “dian” means day). Every cell in our body has an internal clock that allows it to perform different functions during the day and night. A master clock in the center of our brain (the “suprachiasmatic nucleus”) keeps all the clocks in our body aligned with each other and the external environment.

Sleep quality is maximized by sleeping during the time that our 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 two 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 and children in America 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 or school week and leads to dysfunction of the body’s internal clock. Having an unpredictable schedule and a weak circadian rhythm can lead to dysfunction of all the organ systems in our body and is felt to be a contributing factor to obesity, cancer, depression and other health problems.

In summary, our body has an internal clock that functions best when it is given predictable signals from the outside world.

Next Steps:

You can use your new understanding of sleep and wakefulness to think about your own sleep and alertness patterns. Here are some first steps:

  • Think about the times that you wake up during the night. Do they seem to occur every 90 minutes, or in multiples of 90 (after 3 hours of sleep, 4.5 hours, 6 hours or 7.5 hours)? What are you thinking about when you become aware of the awakening? Do you notice that your sleep is less deep as you approach the morning and that you start to have some awareness of your surrounding environment and thoughts even before you wake up? (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?

  • Pay attention to the regularity of your schedule. Are you going to sleep and waking up at the same time on both weekdays and weekends? Are you getting bright light in the morning and dim light at night? Are you eating meals at the same time each day?

About the Author:

I am a board certified Sleep Medicine physician in Redwood City, California. My clinical focus is on the non-pharmacologic management of sleep disorders using techniques such as circadian rhythm optimization, bright light therapy and Cognitive Behavioral Therapy for Insomnia. I believe that understanding the structure of normal sleep and understanding how our ancestors slept in the days before artificial lighting is essential to learning how to sleep better.


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