In the early afternoon, your biological clock sends signals that are similar to (but not as strong as) those it transmits at bedtime. In other words, the longer you’re up, the wearier you become.Īnd that’s not all. Another reason you’re feeling fatigued? Your body builds up adenosine, a chemical that accumulates in your system to cause sleepiness, as the day progresses, says Siebern. Siebern, PhD, consulting assistant professor at Stanford University Sleep Medicine Center and director of the Sleep Health Integrative Program at the Fayetteville VA Medical Center in North Carolina. “ A sense of sleepiness can occur seven to nine hours after our wake up time due the brain’s drop in alertness ,” explains Fitbit sleep advisor Allison T. If you’re longing for a latte by 4:00pm, blame your body clock.