This occurs
because the circadian pacemaker opposes the decrements in
neurobehavioural function associated with increased homeostatic
drive for sleep accumulating with sustained wakefulness. Extension
of the wake episode into the biological night (i.e. past the
evening rise of melatonin) is associated with marked decrements
in neurobehavioural function, because the circadian pacemaker no
longer opposes the wake-dependent deterioration but, instead,
promotes sleep at this circadian phase