<P> In humans , a diurnal variation has been observed dependent on the periods of rest and activity , lowest at 11 p.m. to 3 a.m. and peaking at 10 a.m. to 6 p.m. Monkeys also have a well - marked and regular diurnal variation of body temperature that follows periods of rest and activity , and is not dependent on the incidence of day and night ; nocturnal monkeys reach their highest body temperature at night and lowest during the day . Sutherland Simpson and J.J. Galbraith observed that all nocturnal animals and birds -- whose periods of rest and activity are naturally reversed through habit and not from outside interference -- experience their highest temperature during the natural period of activity ( night ) and lowest during the period of rest ( day ) . Those diurnal temperatures can be reversed by reversing their daily routine . </P>
when does body temperature tend to be lowest
11 p.m. to 3 a.m.