This study investigated a 24-hour variation of subjective mood in 16 healthy Morning-type and 13 Evening-type subjects as defined by the time of day at which their oral temperature curve reached its maximum. The subjects were instructed to use a sleep-wake logbook, in which they kept daily records of the ratings of their mood and alertness for a period of two consecutive weeks. For mood as well as for alertness analysis of variance indicated significant interactions between Morning-and Evening-types and time of day. It is concluded that a pronounced diurnal variation of mood can be observed in healthy individuals, which differs between Morning-type and Evening-type subjects.
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