Circadian rhythms, or our biological clocks, are a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. These rhythms are affected by light and dark or environmental situations such as heat and cold. They are also affected by metabolic, hormonal and neurotransmitter rhythms. Examples include melatonin via the pineal gland, core body temperature and cortisol (the stress hormone).
In addition, artificial lighting such as screens and lamps greatly effect our rhythms. Hence the recommendation to not have TV screens, cellphones or tablets in the bedroom. These can be huge disruptors to proper sleep.
Lights out!