Sleep is an essential function that allows our body and mind to recharge, leaving us refreshed and alert when we wake up. Healthy sleep also helps the body remain healthy and stave off diseases. Without enough sleep, the brain cannot function properly. This can impair our abilities to concentrate, think clearly, and process memories.
Most adults require between seven and nine hours of nightly sleep. Children and teenagers need substantially more sleep, particularly if they are younger than five years of age. Work schedules, day-to-day stressors, a disruptive bedroom environment, and medical conditions can all prevent us from receiving enough sleep. A healthy diet and positive lifestyle habits can help ensure an adequate amount of sleep each night – but for some, chronic lack of sleep may be the first sign of a sleep disorder.
The Science Behind Sleep
An internal “body clock” regulates our sleep cycle, controlling when we feel tired and ready for bed or refreshed and alert. This clock operates on a 24-hour cycle known as the circadian rhythm. After waking up from sleep, we’ll become increasingly tired throughout the day. These feelings will peak in the evening leading up to bedtime.
This sleep drive – also known as sleep-wake homeostasis – may be linked to adenosine, an organic compound produced in the brain. Adenosine levels increase throughout the day as we become more tired, and then the body breaks down this compound during sleep.
Light also influences the circadian rhythm. The brain contains a special region of nerve cells known as the hypothalamus, and a cluster of cells in the hypothalamus called the suprachiasmatic nucleus, which processes signals when the eyes are exposed to natural or artificial light. These signals help the brain determine whether it is day or night.
As natural light disappears in the evening, the body will release melatonin, a hormone that induces drowsiness. When the sun rises in the morning, the body will release the hormone known as cortisol that promotes energy and alertness.
Stages of Sleep
Once we fall asleep, our bodies follow a sleep cycle divided into four stages. The first three stages are known as non-rapid eye movement (NREM) sleep, and the final stage is known as rapid eye movement (REM) sleep.
Stage 1 NREM: This first stage marks the transition between wakefulness and sleep, and consists of light sleep. Muscles relax and your heart rate, breathing, and eye movements begin to slow down, as do your brain waves, which are more active when you are awake. Stage 1 typically lasts several minutes.
Stage 2 NREM: This second NREM sleep stage is characterized by deeper sleep as your heart rate and breathing rates continue slowing down and the muscles become more relaxed. Eye movements will cease and your body temperature will decrease. Apart from some brief moments of higher frequency electrical activity, brain waves also remain slow. Stage 2 is typically the longest of the four sleep stages.
Stage 3 NREM: This stage plays an important role in making you feel refreshed and alert the next day. Heartbeat, breathing, and brain wave activity all reach their lowest levels, and the muscles are as relaxed as they will be. This stage will be longer at first and decrease in duration throughout the night.
