Biorhythms - Physiological functions that vary in a rhythmic way. Most biorhythms are based on a daily, or circadian (24- hour), cycle. Our bodies are governed by an internal clock, which is itself regulated by hormones.
Periods of sleepiness and wakefulness may be affected by the level of melatonin secreted by the pineal gland in the brain.
Melatonin release is stimulated by darkness and suppressed by light. Cortisol, secreted by the adrenal glands, also reflects the sleeping and waking states, being low in the evening and high in the morning.
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