Your body runs on an internal clock, which you can think of as the balance between an alerting signal and sleep drive, as shown in an interactive chart from the sleep division at the Harvard Medical School.
In a healthy schedule, one's sleep drive decreases in the day as one's alerting signal increases.
Stimuli were preceded by a visual spatial signal (to measure the orienting network) and an auditory alerting signal (to measure the alerting network).
A fixation point (+) of a variable duration (400-1600 ms) appeared at the start of each trial, followed by an auditory alerting signal (50 ms duration) in half of the trials (Alerting signal, 2 levels: Presence or Absence of sound).
The alerting index was obtained by subtracting RTs of trials with an alerting signal from those of trials with no alerting signal (but only considering trials with no visual signal).