An hour and a half after waking, early birds and night owls were equally alert and showed no difference in attention-related brain activity. But after being awake for 10 and a half hours, night owls had grown more alert, performing better on a reaction-time task requiring sustained attention and showing increased activity in brain areas linked to attention. More important, these regions included the suprachiasmatic area, which is home to the body’s circadian clock. This area sends signals to boost alertness as the pressure to sleep mounts. Unlike night owls, early risers didn’t get this late-day lift. Peigneux says faster activation of sleep pressure appears to prevent early birds from fully benefiting from the circadian signal, as evening types do.