The missing phrase which describes the the possible relationship between sleep and darkness is the evolutionary psychology and sleep theory.
Based on the theory of evolutionary psychology and sleep theory, it is argued that humans increased ability to sleep at night is based on her intent of reducing its risk exposure which is usually very high at night.
The convergence of humans to assign the night period to sleep comes as a result of the accompanying level of harm which is proportional to dark periods.
Hence, aside the theory of having to save and recoup useful energy to be used during the day, humans adaptive response to predatory risk may be the reason for night sleep patten.
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