We spend about one-third of our lives sleeping. For many animals, sleep represents a time of vulnerability. Some species even developed special defenses against danger from predators while they slept. Dolphins and many species of ducks and birds show “split brain” activity, where one half of the brain sleeps while the other half remains alert for dangers or performs basic movements like swimming or flipper movement to remain afloat.
If sleep is such a dangerous time, then why do we do it?
Research into the purpose of sleep looked first at what happens when sleep is disturbed. Studies have shown correlations between certain sleep disturbances and future health problems. Sleep and dreaming appear to play a role in memory consolidation and learning. Problem-solving may even occur during sleep. Have you ever awakened and found you “magically” have the solution to a problem?
Sleep is not just one long period of inactivity. There’s quite a lot going on to prepare the mind and body for the upcoming day. After a busy day we need to take time to get our ducks in a row for tomorrow.
Stages of Sleep
Sleep occurs in cycles throughout the night; a typical sleep cycle is about 90 minutes long, consisting of two alternating types of sleep: REM (rapid eye movement) and Non-REM (or N-REM).
N-REM sleep is divided into 4 stages. Stage 1 is light sleep, where we can be easily awakened. Stage 2 is the beginning of actual sleep; breathing and heart rate become regular and body temperature begins to drop. Stages 3 and 4 are the deepest and most restorative sleep stages; together they are referred to as “slow-wave” sleep. The REM stage ends each sleep cycle; this is the stage where dreaming normally occurs.
The sleep cycle moves progressively from stage 1 through 4, then progressively back to stage 1 and finally into REM. REM stages become longer and deep sleep stages get shorter as the night progresses. By the end of the night, stages 1 and 2 and REM dominate the sleep cycle.
Quiet (Non-REM) sleep restores the body; REM sleep restores the mind
Sleep and health
Just as food replenishes caloric energy in the body, sleep replenishes nervous system energy. This nerve energy is used by every cell in your body. It governs thought, growth and healing; it powers your brain.
Two general divisions of the nervous system are the sympathetic and parasympathetic divisions. The sympathetic division dominates while we are awake and active. The parasympathetic division is associated with rest and recovery.
Some researchers believe sleep gives the brain a chance to “exercise” neuronal connections to keep them from degenerating or malfunctioning.
Studies with mice indicate sleep is the brain’s maintenance time. Daytime activities lead to a build-up of protein debris in the brain. Some of this (such as beta-amyloid, a protein associated with Alzheimer’s disease) is toxic and some is harmless. While we sleep, the recently discovered glymphatic system clears out debris in the brain. (The lymphatic system that clears waste from the body cannot access the brain.) Without this clearing in the brain we risk neurodegenerative disease.
Deep sleep is when healing takes place. Sleep helps in healing and repair of the heart and blood vessels. Chronic sleep deficiency raises the risk of heart or kidney disease, high blood pressure or stroke.
In one study, subjects were given paper cuts and then monitored while they slept. One group was prevented from entering deep sleep; their paper cuts did not heal. The cuts healed well for the control group which was allowed deep sleep.
Sleep also affects the endocrine (hormone) system. Ghrelin (the hormone that makes you feel hungry) and leptin (the hormone that makes you feel full) are influenced by sleep. And sleep affects how your body reacts to insulin; sleep deficiency increases the risk of diabetes.
Deep sleep is when most growth hormone is released to help repair tissues and cells. Poor sleep means less slow wave, deep sleep time. (Increased cortisol slows growth hormone production even more.) Human growth hormone is your natural “anti-aging” weapon.
Sleep and the immune system
The neurons that control sleep also interact with the immune system. When the immune system produces cytokines to fight infections, those same chemicals induce sleep. That’s why you feel drowsy when you are fighting the flu or an infection. Sleep lets the body conserve energy the immune system needs to do its healing work.
Sleep and Weight Control
According to the CDC more than 35% of the population is sleep-deprived; that’s the same as the statistic for obesity. Studies with dieters showed that with adequate sleep, the weight lost on a particular diet was half fat. When dieters on the same diet were deprived of sleep, the weight loss was less than half fat, and the subjects felt hungry and had less energy to exercise.
Researchers at the University of Chicago found that just four days of sleep deprivation reduces insulin sensitivity by more than 30 percent. Insulin resistant people store more fat.
Other studies found that sleeping less than 6 hours a night affects production of leptin and ghrelin, the two hormones that control hunger. You feel hungrier and less satisfied after a meal, your fat-burning metabolism is reduced, and you store more fat. Cortisol levels are increased, too; cortisol is a hormone associated with stress and increased fat storage.
Yet another study found that just one night of sleep deprivation affected frontal lobe activity in the brain; the frontal lobe controls decision-making. So if you feel hungry and your decision-making ability isn’t at 100%, what kind of food will you choose? Sleep deprivation can destroy diets.
Lack of sleep also decreases protein synthesis – that’s “muscle making” – in the body, and muscle loss not only slows down fat-burning but also leads to injuries.
Sleep, memory and learning
Sleep helps enhance problem-solving skills and general learning.
There are two ways sleep and learning are connected:
- Sleep deprived people have difficulty concentrating and learning efficiently; and
- Sleep itself is directly involved in consolidation of memories, which is essential to learning.
Three functions are involved in learning: acquisition of information, consolidation of the memory, and later recall of the information. Acquisition and recall take place when we are awake; consolidation occurs during sleep. Researchers believe consolidation is accomplished through strengthening of neural connections.
Different types of learning seem to be consolidated during different stages of sleep. One theory holds that the characteristics of specific brainwaves during sleep indicate formation of particular types of memory.
Declarative memory refers to fact-based information; both REM and slow-wave sleep appear to be involved in learning facts, depending on their complexity. Procedural memory refers to knowing how to do something; REM sleep seems to be critical here. Learning motor skills appears to depend on lighter sleep stages, while visual learning depends on the amount and timing of both REM and slow-wave sleep.
In one study reported by the National Institutes of Health (NIH) when people learning a new skill were deprived of N-REM sleep they could still recall what they had learned, but people deprived of REM sleep could not.
Sleep is when the body and mind recover from one day’s activity and prepare for the next. One of the world’s leading sleep researchers, Dr. Peretz Lavie, says we can live longer without food than we can without sleep.
So, go ahead! Line up those ducks for tomorrow!
www.health101.org/art_Sleep.htm; Sleep, the Great Healer by Don Bennett
www.newyorker.com/science/maria-konnikova/why-we-sleep; Maria Konnikova; The Work We Do While We Sleep, July 8, 2015