Web Resources for Circadian nadir
Relevant Clinical Literature
Wikipedia on Circadian nadir (Less technical, ? quality control)
UpToDate® on Circadian nadir (Technical, SUBSCRIPTION)
The period a few hours before going to sleep when hormonal influences maximise to keep you awake. So if your normal bedtime is 10pm trying to get to sleep about 6pm could be problematical due to your physiology.