Sleep habits can affect hormone peaking. Some people have more flexible rhythms and adjust better to unusual work schedules and are good candidates for space missions. Age affects flexibility; it is less easy to adjust in middle age.
Larks feel lively in the morning; body temperature cycles peak early in the day with large differences between top and bottom of the cycle. Suffer more from disturbed sleep if they work at night and when they fly across time zones from east to west. Owls have body cycles that peak later in the day than larks.
Every person has their own typical sleep length, around 8 hours. Sleep has two components - duration and placement. On free schedule study, people slept in fragments, with night time sleep periods typically double the length of day time ones. Between 7am and 2pm or 3pm a 'sleepability gate' opens every 90 to 120 minutes; after 3pm a wide sleepability gate opens and if you have a siesta during this period it is easier to wake from REM sleep than during the night. REM (rapid eye movements) sleep blocks muscle movements and learning memeory and attention stop (impossible to learn while asleep).
Jeremy Campbell: Winston Churchill's Afternoon Nap.
