Normal sleep | Sleep Center | Salem Health
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What is normal sleep?

Sleep is regulated by two body systems:

  • Sleep/Wake Restorative Process: Balances sleep and wakefulness
  • Circadian Biological Clock: Regulates timing of sleep and wakefulness

When you sleep, when you wake, and your alertness levels are all controlled by your circadian rhythm.


Circadian Rhythm is an internal clock within us that coordinates our daily cycles
This cycle is driven at the cellular level, clearly something we can not easily change.

Scientists believe that the Body Clock resides in the Suprachiasmatic (‘SUP-ra - ky-asth-MA-tic’) Nucleus of the hypothalamus.

Over 352 internal body functions are controlled by this daily cycle. Internal core body temperature varies during a 24-hour cycle and is associated with circadian rhythms. So is our capacity for digestion. Your immune system is more active during the daylight hours

The top graph shows two circadian cycles over 48 hours and some of the physiologic changes.

Most mammals, including us are diurnal and have circadian rhythms that around 24 hours long matching the earth going around the sun.

Most powerful is stimulant for wakefulness is sunlight especially but artificial
Light can simulate wakeful properties of natural sunlight.

We also have an internal hormone, melatonin, which is excreted by our pineal
gland at night and which is also synchronized to the light/dark cycle by our
circadian clock. There are melatonin receptors in the brain.

The graph below shows a single 24 cycle in more detail. This is a graph of the average circadian rhythm or alertness level of an individual who wakes at approximately 6:00 a.m.

Around this time, your body will release hormones such as cortisol and adrenaline to increase your alertness which peaks between 9 and noon, and then around 1500, another dose of the hormones will be released and again the alertness level will be up for a while in the evening. Then it wears off to allow our bodies to sleep during the night.

Likewise, there are two noticeable troughs or dips in our alertness and increased sleepiness that undoubtedly decreases performance.

The most dramatic one is between midnight and 5:00 a.m, the time when most are naturally asleep but many unfortunately have to work through a night shift. The other is what some call the “post-lunch” dip between 1:00-5:00 p.m. Clearly during these troughs we can expect a significant decline in our performance.


An important point is that we cannot easily change this cycle so when we try to work different shifts or double our shift and work through the night, we can expect a marked decrease in performance.

Attempting to change this cycle can also significantly interfere with our ability to get quality sleep. We have all been there, trying to sleep when you are naturally alert!

When we are constantly bombarded with factors that work against us getting the sleep we truly need.

For most of us, our circadian rhythm is reset on our days off as we sleep in or go from night shift to a day shift so we can spend time with our family or get those necessities done, like going to the bank or grocery shopping.

Individual circadian rhythms are unique.  Owls are people who don’t feel tired until midnight or latter and then sleep until noon.  Larks get to bed early and wake up at the crack of dawn.  Good news for owls, they can handle changes in their circadian rhythm significantly better.

Unfortunately, whether an owl or a lark, as we age it is more difficult to adjust / recover from circadian disruptions. You may have found you handled the night shift with no problems when you were young, but you may find that later in life you realize you just can’t work at night and sleep during the day.

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