I have been sleeping only when tired, and getting up when I feel awake (not forcing myself back to sleep). I have fallen into a biphasic sleep pattern of sleeping for 4-5 hours plus an additional, shorter sleep (1 cycle).
Normally I sleep from 9pm-1am if I am sleepy early, or 11pm-3am/4am if later. I don't use an alarm clock and I turn on a soft-light lamp at night (and my monitor brightness is very low). When I wake up I am active in these low light conditions.
Monophasic sleep is assumed to be "normal" for humans (among the medical and scientific community), but I think biphasic sleeping produces higher levels of alertness, requires less time and may be more natural (the result of free running sleep).
An excellent article on sleep:
http://www.supermemo.com/articles/sleep.htmScroll down for the section on
free running sleep. Interestingly, the log suggests that wakefulness/sleepiness can occur at anytime of day. The sleeper had a long "core" sleep time and a very short siesta. My sleep of late has been the reverse. A short core with a longer siesta, and fewer hours overall. If I miss my siesta, my core sleeping time will be longer, but it is never more than 6 hours. Falling asleep never takes more than a minute or two.
Also notable in the article is the section on the physiology of sleep and the mention of Francis Crick's theory that the purpose of REM sleep is
reverse learning (i.e. forgetting).
The 21 myths on sleep made me grin.