Diurnal Variations Of Mood
Diurnal variations of mood refers to the human tendency to have shifting moods throughout the 24 hour day/night cycle. The basic cycle for humans is be awake and active during the day and to be asleep at night.
Within that basic cycle are variations with some people feeling active and energetic in the morning, while others feel "up" at later times during the day. These diurnal variations are called chronotypes with the spectrum ranging from 'larks' or individuals with 'morningness' (those who wake early and are most alert then) and 'owls' or individuals with 'eveningness' (those who are more alert later in the day and prefer to stay up late).
In some cases diurnal variations can switch to where a person may be awake at night and sleepy during the day. While this can be of benefit to those who work night shifts, for most people it causes inconvenience and may need medical intervention to change this cycle.