What happens when you sleep?
When you sleep, your body rests and restores its energy levels. However, sleep is an active state that affects both your physical and mental well-being. A good night's sleep is often the best way to help you cope with stress, solve problems, or recover from illness. Sleep is prompted by natural cycles of activity in the brain and consists of two basic states: rapid eye movement (REM) sleep and non-REM (NREM) sleep, which consists of 4 stages.
During sleep, the body cycles between non-REM and REM sleep. Typically, people begin the sleep cycle with a period of non-REM sleep followed by a very short period of REM sleep. Vivid dreams tend to occur during REM sleep.
What is REM sleep?
The period of REM sleep is marked by extensive physiological changes. These include:
- Accelerated respiration
- Increased brain activity
- Eye movement
- Muscle relaxation
Usually, REM sleep occurs 90 minutes after sleep onset. The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one lasting an hour. Polysomnograms (sleep readings) show wave patterns in REM to be similar to stage 1 sleep. In people without sleep disorders, heart rate and respiration speed up and become erratic during REM sleep. The face, fingers, and legs might twitch. Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but paralysis occurs simultaneously in the major voluntary muscle groups. REM is a mixture of encephalic (brain) states of excitement and muscular immobility. For this reason, it is sometimes called paradoxical sleep.
The percentage of REM sleep is highest during infancy and early childhood. During adolescence and young adulthood, the percentage of REM sleep declines, and the percentage decreases further in older age. In many cases, older people enter REM sleep more quickly and remain in REM sleep longer.
What is NREM sleep?
The period of NREM sleep is made up of stages 1 to 4. Each stage can last from five to 15 minutes. Stages 2 and 3 repeat backwards before REM sleep is attained.
Polysomnography shows a 50 percent reduction in activity between wakefulness and stage 1 sleep. The eyes are closed during stage 1 sleep. However, if aroused from this stage of sleep, a person might feel as if he or she has not slept. Stage 1 might last for five to 10 minutes.
This is a period of light sleep during which polysomnographic readings show intermittent peaks and valleys, or positive and negative waves. These waves indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and the body temperature decreases. At this point, the body prepares to enter deep sleep.
Stages 3 and 4
These are deep sleep stages, with stage 4 being more intense than stage 3. These stages are known as slow-wave, or delta, sleep.
During NREM sleep, the body repairs and regenerates tissues, builds bone and muscle, and appears to strengthen the immune system. As you get older, you get less NREM sleep. People under age 30 have about two hours of restorative sleep every night, while those over 65 might get only 30 minutes.