rsquo,s biological rhythm. It comprises 24 hours roughly split between 16 hours wakefulness and 8 hours sleep and can therefore also be called the sleep-wake cycle. This rhythm is influenced directly by daylight. The release of the hormones cortisol and melatonin are controlled by the circadian rhythm. Cortisol secretion is at its highest in the morning making us more productive. While the hormone melatonin decreases steadily with increasing daylight brightness, so we feel fit and well-rested during the day and become more tired as daylight decreases helping us to sleep better at night.nn