In the brain, circadian rhythms are regulated by the expression of several genes. These genes code for the proteins PER and TIM which build up over time and at high levels cause the animal to . After a certain point, these proteins inhibit their own production, causing levels to decrease and the animal to

Sleep.

Awaken.

Explanation:

The time and duration of circadian rhythms  are regulated  by the interactions of PER(period) and TIM(timeless) genes and their proteins. The TIM proteins  binds with PER- proteins and the two entered the cell nucleus to set up  circadian rhythms.

At midday the two genes becomes active,and their DNAs transcribed to form PER-RNA, and TIM -RNA. These RNAs accumulated in the cell nucleus   over some hours.

At dusk(night) the  amount  of these  RNAs reached highest levels.and are translated  to the PER and TIM -proteins, which bonded together and moved into the  cells nucleus causing  sleep in the animals.

Hours to dawn,(morning) the concentration of the two proteins exceeded their limit, and signals is sent to their genes to stop transcription, thus the protein(PER  and TIM) synthesis is stopped.

At almost dawn, the PER and TIM proteins disintegrates, the cycle begins again and the individual is awake.

your body is trying to warm itself up. or more precisely, sensors in your skin send messages to your brain to generate heat to your body.