Our body is taking care of us, also by reminding us of when to go to sleep.
Our body has two internal biological mechanisms: CIRCADIAN RHYTHM (Process C) and HOMEOSTASIS (Process S); that work together to regulate time when you are awake and sleep. So, sleep timing is controlled by these two processes and to some extent also by the individual will regarding sleep, sometimes enforced by society or individual itself.
CIRCADIAN RHYTHM / CLOCK (PROCESS C)
CIRCADIAN RHYTHMS are responsible for a wide variety of functions, they direct daily fluctuations in wakefulness, regulate body temperature, metabolism, and the release of hormones. They control our timing of sleep, cause us to be sleepy at night and make the tendency to wake up in the morning without an alarm. Circadian rhythms synchronize with environmental cues (light, temperature) about the actual time of day, but they continue even in the absence of cues.
Our body’s biological clock is based on a 24-hour day and controls most circadian rhythms. As already mentioned, circadian rhythms affect body temperature (between roughly 36.2 °C and 37.2 °C), which is represented as the white line on the chart above. It also affects melatonin – a hormone released by the pineal gland, which helps you feel sleepy once the lights go down. The peaks and valleys of melatonin, represented as the gold line above, are important for matching the body’s circadian rhythm to the external cycle of light and darkness.
Cortisol levels typically rise throughout the night, peak in the awakening hours and diminish during the day. Circadian prolactin secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone.
The circadian rhythm influences the ideal timing of a restorative sleep episode. Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereas slow-wave sleep can occur more independently of circadian time.
The internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, at the right moment in the circadian cycle, can significantly ‘reset’ the internal clock. Blue light exerts the strongest effect, leading to concerns that use of a screen before bed may interfere with sleep.
Modern people can often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially night shifts), long-distance travel, and the influence of universal indoor lighting. Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle and then they can have difficulty waking up in the trough of the cycle.
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. A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise.
HOMEOSTASIS (PROCESS S)
Generally speaking, the longer an organism is awake, the more it feels a need to sleep (“sleep debt”). This sleep drive is referred to as PROCESS S or SLEEP-WAKE HOMEOSTASIS, which keeps track of our need for sleep, as the balance between sleeping and waking is regulated by a process called homeostasis.
Induced or perceived lack of sleep is called sleep deprivation.
Process S is driven by the depletion of glycogen and accumulation of adenosine in the forebrain that disinhibits theventrolateral preoptic nucleus, allowing for inhibition of the ascending reticular activating system.
Sleep deprivation tends to cause slower brain waves in the frontal cortex, shortened attention span, higher anxiety, impaired memory, and a grouchy mood. Conversely, a well-rested organism tends to have improved memory and mood. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.
There is disagreement on how much sleep debt is possible to accumulate, and whether sleep debt is accumulated against an individual’s average sleep or some other benchmark. It is also unclear whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. Sleep debt does show some evidence of being cumulative. Subjectively, however, humans seem to reach maximum sleepiness 30 hours after waking. It is likely that in Western societies, children are sleeping less than they previously have.
One neurochemical indicator of sleep debt is adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine levels increase in the cortex and basal forebrain during prolonged wakefulness, and decrease during the sleep-recovery period, potentially acting as a homeostatic regulator of sleep. Coffee, tea, and other sources of caffeine temporarily block the effect of adenosine, prolong sleep latency, and reduce total sleep time and quality.
Factors that influence your sleep-wake needs include also medical conditions, medications, stress, sleep environment, and what you eat and drink, but 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.
SOCIAL TIMING
Humans are also influenced by aspects of social time, such as the hours when other people are awake, the hours when work is required, the time on clocks, etc. Time zones, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. An extreme example of the approximate nature of time zones is China, a country which used to span five time zones and now officially uses only one.
As you can see, our bodies are incredibly smart and are all the time striving to keep us healthy, but it is on us if we will listen to our bodies and stay healthy or not.