The reason we stay awake and sometimes feel sleepy even when we are not tired is due to a substance called:

Explanation

Adenosine accumulates in the brain during prolonged wakefulness and contributes to “sleep pressure,” making a person increasingly sleepy. Caffeine promotes alertness largely by blocking adenosine receptors. Strictly speaking, adenosine is better described here as a neuromodulator rather than a conventional endocrine hormone.