Sleep Care

What Happens Inside Our Bodies While We Sleep?

When we get tired or drowsy, what is going on inside our bodies to cause these feelings?  How does our body functioning change when we sleep, compared to when we’re awake?

The answers to these questions start with a basic understanding of sleep, which is split into alternating periods of NREM (Non Rapid Eye Movement) and REM sleep.  NREM sleep starts off being fairly light and easy to wake from.  It becomes deeper before transitioning to REM sleep, which is where most dreaming takes place.  The overall process (one period of NREM and REM sleep) is called a sleep cycle and takes about 90-120 minutes to complete.  Early in the night, sleep cycles are comprised primarily of NREM sleep, but this changes over the course of the night so that REM sleep is the majority of the cycle in the last few hours before waking up in the morning.  This is why people are generally more likely to remember dreams when getting up for the day, compared to waking up in the middle of the night.

NREM Sleep

NREM sleep is actually composed of three distinct stages.  The first stage—called N1, for NREM 1—is characterized by brain waves (also known as EEG, for electroencephalogram) that are slightly slower than those during wake.  This is because the brain is beginning to slow down for sleep.  Stage N1 is brief—usually about 10% of sleep during the night.  It often serves as more of a “transition” to where the brain spends the majority of its time—up to 50%–at night: the second stage of sleep, called Stage N2.

In Stage N2 sleep, the brain shows two distinct waveforms that are specific to this stage: K-complexes and sleep spindles, which are mini bursts of activity.  Uninterrupted Stage N2 sleep will usually yield to Stage N3, where the brain is at its deepest state of rest during the night.  Brain activity is slow because of this rest.  Stage N3 typically takes up about 15% of the total sleep time during the night.  It also has the highest auditory threshold of all the different stages of sleep, meaning that people in Stage N3 are particularly difficult to wake up using methods like calling their name out loud or having an alarm clock go off.

There are various changes in different body systems that occur when the body is in NREM sleep.  In the nervous system, there is a reduced neural discharge rate and brain metabolism, as the brain is mainly resting.  From a cardiovascular standpoint, the overall heart rate, cardiac output, and blood pressure are all lowered (in the absence of any sleep disorder that might fragment sleep, such as sleep apnea.)  Breathing rates also tend to be slightly lower in NREM sleep.  Increased production of human growth hormone occurs during Stage N3 sleep—part of the reason why significant growth happens during the years when this stage of sleep occurs the most (infant up through adolescence).

REM Sleep

Does the body respond differently to the dreams of REM sleep than the other stages of sleep?  While there are some notable differences, there are numerous other body changes that occur throughout the night as a whole. Urine production typically decreases—to help avoid disruptions of sleep from getting up to go to the bathroom—but the concentration of urine increases.  Also, the swallowing reflux slows during sleep.  For those with problems drooling onto their pillow throughout the night, this may be the explanation.

Compared to NREM sleep, REM is a very active stage that accounts for about 20-25% of the total sleep time during the night.  Most dreaming occurs during this stage, making the brain EEG very active and somewhat similar to wake or stage N1 EEG.  In addition to the rapid eye movements for which this stage of sleep is known, another distinguishing characteristic is muscle atonia—meaning that most of the body’s muscles, such as the arms and legs, are paralyzed to prevent the body from physically acting out whatever the brain is dreaming about.  Some individuals who wake too quickly out of REM sleep experience sleep paralysis, which occurs when an individual becomes consciously awake before the chemicals responsible for the muscle atonia have completely worn off.

In REM, cells in an area of the brain called the hypothalamus stop firing, causing the body’s ability to regulate its internal temperature to cease.  This results in the body’s temperature becoming as hot or as cool as the surrounding environment.  An interesting consequence of this is that at extreme temperatures—very hot or very cold conditions—REM sleep can be lost entirely.

Overall cerebral blood flow during REM sleep is high, which makes sense given how active the brain is during dreaming.  Areas of the brain related to visual imagery and memory are particularly active.  Overall neural firing rates and brain metabolism rates are increased.

Blood pressure and heart rates are generally elevated in REM sleep, particularly when the eye movements occur (which are known as phasic events).  Other phasic events that can occur during REM are short bursts of muscle activity, often in the chin and jaw (although they can sometimes occur in the arms and legs as well if the body fails to enter a sufficient state of muscle atonia).

Breathing rates in REM become more variable compared to NREM stages of sleep.  This may be in response to the various images and events occurring in dreams while people are in REM.  Overall, breathing rates are faster, despite the increased variability.