When you ’re busy and on the go, you ’ll naturally start getting tired as the day wears on. That’s impeccably normal — in fact, that’s how our bodies work.

“ We've a sleep drive that builds up the longer we ’re awake, ” says sleep drug expert Michelle Drerup, PsyD, DBSM. “ By the end of the day, that sleep drive — or pressure for sleep — is high. ”
Still, each of us has what’s called an “ internal sleep meter, ” or circadian meter. This meter varies from person to person, and represents the time when our body and mind start relaxing and winding down for the day.