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Your Biological Clock Explained
Circadian rhythms influence the 24 hour sleep/wake cycle, hormone release, body temperature and other biological activities. They are controlled by our biological clocks. Humans are diurnal creatures, meaning we have a natural tendency to be wakeful by day and sleep at night. Studies of rats conducted in the 1970s determined that the biological clock is located in a part of the brain called the hypothalamus, within a cluster of nerve cells called the suprachiasmatic nucleus (SCN). When this area was damaged, rats lost their ability to regulate their sleep/wake cycles. The hypothalamus is located where the optic nerves from the eyes cross. The biological clock is responsive to several external factors called zeitgebers (from the German “time giver”) to keep it ticking over in 24 hour intervals. They are as follows:
Light
Light striking the eyes sends messages through the optic nerves to the hypothalamus. Light is a signal for the brain to become alert, which is why bright lights before bedtime can make it hard to wind down and sleep.
Light striking the eyes sends messages through the optic nerves to the hypothalamus. Light is a signal for the brain to become alert, which is why bright lights before bedtime can make it hard to wind down and sleep.
Melatonin production
Melatonin is a hormone produced in the pineal gland. The suprachiasmatic nucleus plays a role in melatonin production, as it contains receptors for the hormone. Melatonin levels rise and fall in in a predictable pattern with night and day and are thought to play a role in the sleep/wake cycle. Some people find taking a melatonin supplement helps them to sleep.
Melatonin is a hormone produced in the pineal gland. The suprachiasmatic nucleus plays a role in melatonin production, as it contains receptors for the hormone. Melatonin levels rise and fall in in a predictable pattern with night and day and are thought to play a role in the sleep/wake cycle. Some people find taking a melatonin supplement helps them to sleep.
Temperature
This is believed to be a factor, although reduced temperatures are likely to correspond with times of darkness, so the influence of this variable is hard to determine.
This is believed to be a factor, although reduced temperatures are likely to correspond with times of darkness, so the influence of this variable is hard to determine.
Cognitive factors
By having a daily schedule full of tasks and activities, the brain is to some extent compelled to remain alert by these external stimuli.
Other hormones affected by circadian rhythms:
Cortisol
Sometimes referred to as “the stress hormone” cortisol plays a role in the formation of blood glucose and supports anti-stress and anti-inflammatory actions in the body.
Sometimes referred to as “the stress hormone” cortisol plays a role in the formation of blood glucose and supports anti-stress and anti-inflammatory actions in the body.
Growth hormone
This critical hormone which supports growth and restorative processes in the body is secreted only during sleep.
This critical hormone which supports growth and restorative processes in the body is secreted only during sleep.
Thyrotropin (thyroid stimulating hormone)
This hormone is suppressed during sleep. Thyrotropin stimulates the thyroid gland to produce thyroxine, which stimulates metabolism.
This hormone is suppressed during sleep. Thyrotropin stimulates the thyroid gland to produce thyroxine, which stimulates metabolism.
For most people, the strongest tendency for sleepiness is felt between approximately 2am – 3am, and again between 2pm and 3pm. These lulls in alertness are known as “circadian dips.”
In countries where a mid-afternoon siesta is the norm, people can harmonise their sleeping patterns with their internal biological rhythms.
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