Early birds wake up at the crack of dawn and struggle to stay alert and productive (especially in the cognitive realm) in the evening. Night owls perform well in the evening but are worthless if you yank them out of bed too early in the morning. There are less-well-known differences, too: early birds experience what scientists call “a faster build-up of homeostatic sleep pressure” during the day compared to night owls, who, like a certain battery-powered bunny, just seem to keep going and going, resisting the pressure to sleep. (That must be why I practically turn into a pumpkin by 9 p.m.) And when they do sleep, early birds experience a faster dissipation of that sleep pressure, feeling restored more quickly than night owls. Now a new study, in the journal Science, reports some intriguing differences between the brain-activity patterns of the two types that underlie the behavioral differences.

Scientists led by Christina Schmidt and Philippe Peigneux of the University of Liege in Belgium had 15 extreme night owls and 16 extreme early birds spend two nights in a sleep lab. The two groups were separated by about four hours in their sleep patterns; if early birds were happy waking up at 7, night owls slept til 11, and early birds were ready to go to sleep at 11 while night owls had no trouble staying up til 3 in the morning. An hour and a half after waking up, and again 10.5 hours after waking up, the volunteers had their brain activity measured by fMRI while they took a simple reaction-time test of their ability to maintain focused attention. Both the early birds and the night owls were sleeping and waking whenever they pleased, rather than being kept on an artificial schedule.

There was no real difference between the early birds and the night owls in their performance on the morning test. But the evening test was a different story: night owls were less sleepy and had faster reaction times than early birds. (Just to emphasize, 'evening' was a relative term: it was a different actual time for each group, but the exact same 10.5-hours-after-waking for both early birds and night owls.) So even though both groups were sleeping and waking according to their preferred schedule, night owls generally outlasted early birds in how long they could stay awake and mentally alert before becoming mentally fatigued. The fMRI supported the behavioral results: 10.5 hours after waking up, the early birds had lower activity in brain regions linked to attention and the circadian master clock, compared to night owls.