Baboons are diurnal, meaning they are awake during the day and sleep overnight — just like humans.
However, they don't tend to stay up late browsing the internet, so they probably go to bed earlier than we do, and rise when it starts to gets light.
When it starts to get dark, a chemical (melatonin) in the baboon's brain will signal that it is time to go to sleep. Humans would do the same, if we didn't have schedules to keep and such busy lives.
A baboon doesn't have to worry about under- or over-sleeping, and so there's no need for it to ignore the drowsiness brought on by increased melatonin production. We, on the other hand, are into the habit of staying awake long after we start getting tired!