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 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.
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.
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:
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.
This critical hormone which supports growth and restorative processes in the body is secreted only during sleep.
This hormone is suppressed during sleep. Thyrotropin stimulates the thyroid gland to produce thyroxine, which stimulates metabolism.