You're not getting enough sleep

Caffeine, even in the afternoon, can interfere with your sleep. Even small amounts of sleep loss can add up and disturb your daytime alertness and performance.

Using caffeine to mask sleep deprivation can create an unwelcome cycle. For example, you may drink caffeinated beverages because you have trouble staying awake during the day. But the caffeine keeps you from falling asleep at night, shortening the length of time you sleep.