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Napping and Sleep Rythem Science

By Dan LaVine | 11.11.2019

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Daytime napping helps to restore one’s energy and alertness.

Although it is common for people in many western societies to sleep in a single consolidated block of about eight hours during the night, this is by no means the only sleep pattern. In fact, following this schedule and foregoing an afternoon nap would seem highly abnormal to many people around the world.

In many cultures, particularly those with roots in tropical regions, afternoon napping is commonplace and is built into daily routines. And although the exact timing of naps is not officially scheduled, it is not uncommon for stores and government offices to close and for many activities to stop for an hour or two every afternoon.

Afternoon nap time typically coincides with a brief lag in the body's internal alerting signal. This signal, which increases throughout the day to offset the body's increasing drive to sleep, wanes slightly in mid-afternoon, giving sleep drive a slight edge. Napping also typically happens during the warmest period of the day and generally follows a large mid-day meal, which explains why afternoon sleepiness is so often associated with warm afternoon sun and heavy lunches.

Afternoon naps for most people typically last between 30 and 60 minutes. Any longer and there is a risk of falling into deep sleep and having a difficult time waking. Following a nap, having dissipated some of the accumulated sleep drive, many people report feeling better able to stay awake and alert in the late afternoon and evening. This increased alertness typically causes people to go to bed later and generally to sleep less at night than people who do not take naps.

According to sleep experts, napping can be a good way for people who do not sleep well at night to catch up. They do caution, however, that people with insomnia may make their nighttime sleep problem worse by sleeping during the day. Otherwise, they generally recommend naps for people who feel they benefit from them.

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Sleep mechanisms

Two internal biological mechanisms–circadian rhythm and homeostasis–work together to regulate when you are awake and sleep.

Circadian rhythms direct a wide variety of functions from daily fluctuations in wakefulness to body temperature, metabolism, and the release of hormones. They control your timing of sleep and cause you to be sleepy at night and your tendency to wake in the morning without an alarm. Your body’s biological clock, which is based on a roughly 24-hour day, controls most circadian rhythms. Circadian rhythms synchronize with environmental cues (light, temperature) about the actual time of day, but they continue even in the absence of cues.

Sleep-wake homeostasis keeps track of your need for sleep. The homeostatic sleep drive reminds the body to sleep after a certain time and regulates sleep intensity. This sleep drive gets stronger every hour you are awake and causes you to sleep longer and more deeply after a period of sleep deprivation.

Factors that influence your sleep-wake needs include medical conditions, medications, stress, sleep environment, and what you eat and drink. Perhaps the greatest influence is the exposure to light. Specialized cells in the retinas of your eyes process light and tell the brain whether it is day or night and can advance or delay our sleep-wake cycle. Exposure to light can make it difficult to fall asleep and return to sleep when awakened.

Night shift workers often have trouble falling asleep when they go to bed, and also have trouble staying awake at work because their natural circadian rhythm and sleep-wake cycle is disrupted. In the case of jet lag, circadian rhythms become out of sync with the time of day when people fly to a different time zone, creating a mismatch between their internal clock and the actual clock.
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Now, here's the science behind it all;

Anatomy of Sleep

Several structures within the brain are involved with sleep.

The hypothalamus, a peanut-sized structure deep inside the brain, contains groups of nerve cells that act as control centers affecting sleep and arousal. Within the hypothalamus is the suprachiasmatic nucleus (SCN) – clusters of thousands of cells that receive information about light exposure directly from the eyes and control your behavioral rhythm. Some people with damage to the SCN sleep erratically throughout the day because they are not able to match their circadian rhythms with the light-dark cycle. Most blind people maintain some ability to sense light and are able to modify their sleep/wake cycle.

The brain stem, at the base of the brain, communicates with the hypothalamus to control the transitions between wake and sleep. (The brain stem includes structures called the pons, medulla, and mid brain.) Sleep-promoting cells within the hypothalamus and the brain stem produce a brain chemical called GABA, which acts to reduce the activity of arousal centers in the hypothalamus and the brain stem. The brain stem (especially the pons and medulla) also plays a special role in REM sleep; it sends signals to relax muscles essential for body posture and limb movements, so that we don’t act out our dreams.

The thalamus acts as a relay for information from the senses to the cerebral cortex (the covering of the brain that interprets and processes information from short- to long-term memory). During most stages of sleep, the thalamus becomes quiet, letting you tune out the external world. But during REM sleep, the thalamus is active, sending the cortex images, sounds, and other sensations that fill our dreams.

The pineal gland, located within the brain’s two hemispheres, receives signals from the SCN and increases production of the hormone melatonin, which helps put you to sleep once the lights go down. People who have lost their sight and cannot coordinate their natural wake-sleep cycle using natural light can stabilize their sleep patterns by taking small amounts of melatonin at the same time each day. Scientists believe that peaks and valleys of melatonin over time are important for matching the body’s circadian rhythm to the external cycle of light and darkness.

The basal forebrain, near the front and bottom of the brain, also promotes sleep and wakefulness, while part of the midbrain acts as an arousal system. Release of adenosine (a chemical by-product of cellular energy consumption) from cells in the basal forebrain and probably other regions supports your sleep drive. Caffeine counteracts sleepiness by blocking the actions of adenosine.

The amygdala, an almond-shaped structure involved in processing emotions, becomes increasingly active during REM sleep.

Credit;
http://healthysleep.med.harvar...

https://www.ninds.nih.gov/Diso...