Did you ever wonder why more people fall asleep on the wheel of a car in the early morning or mid afternoon than in the evening?
This is as a result of the mechanism of the sleep-wake cycle, our body is programmed by our biological clock to experience two natural periods of sleepiness during the 24-hour day, regardless of the amount of sleep we've had in the previous 24 hours.
The primary period is between about midnight and 7:00 a.m. A second period of less intense sleepiness is in the mid afternoon, between about 1:00 and 3:00. This means that we are more at risk of falling asleep at the wheel at these times than in the evening, especially if we haven't been getting enough sleep!
This 24-hour cycle is controlled by the Circadian rhythms that refer to the cyclical changes, like fluctuations in body temperature, hormone levels, and sleep that occurs over a 24-hour period, driven by the brain's biological "clock". These internal 24-hour rhythms in physiology and behavior are synchronized to the external physical environment and social/work schedules.
Light and darkness are external signals that "set" the biological clock and help determine when we feel the need to wake up or go to sleep. In addition to providing synchronization in time between various rhythms, the circadian clock also helps promote wakefulness.
Our circadian rhythm (or 'body clock') is influenced by melatonin as it acts as a signal, anticipating sleep. It is also an important physiological regulator of the sleep-wake cycle1.
Melatonin (which is a naturally occurring hormone that is key to sleep and the sleep-wake cycle in humans and animals) is the marker for circadian rhythms1. In the body melatonin is produced by the pineal gland in the brain when night falls. The retina detects failing light, and the level of melatonin gradually increases, reaching its peak in the middle of the night in natural circumstances2.
In the morning when plasma levels of melatonin decrease, sleep ends and wakefulness begins. Melatonin is not sedative, but a sign of darkness or night time and is a signal for innate night time behavior. In the morning when plasma levels of melatonin decrease, sleep ends and wakefulness begins1. In nocturnal animals, melatonin stimulates wakefulness. In humans, melatonin induces heat loss, reduces arousal and related brain activity and delays production of cortisol, which increases blood pressure and blood sugar in preparation for sleep3.
Studies have shown that the increase in prevalence of sleep disorders associated with age is reflected by the decrease in melatonin levels that occur with age4.
