Sleep drive, also known as sleep pressure, refers to the body’s biological need for sleep that builds up gradually during the hours of wakefulness. The longer a person has been awake, the stronger the urge to sleep becomes. This accumulated need for sleep is one of the two primary forces — alongside the circadian rhythm — that regulate when we feel sleepy and when we feel alert. Sleep drive is regulated by the accumulation of adenosine, a chemical byproduct of brain activity, in the brain during wakefulness. As adenosine levels rise throughout the day, the feeling of sleepiness increases. During sleep, adenosine is cleared from the brain, reducing sleep drive and allowing the person to feel refreshed upon waking. The interaction between sleep drive and the circadian rhythm determines the optimal timing for sleep. The circadian rhythm creates a natural alerting signal that counteracts sleep drive during the day, helping us stay awake. As the evening approaches and the alerting signal weakens, sleep drive takes over, producing the familiar feeling of tiredness at bedtime. Caffeine works by temporarily blocking adenosine receptors in the brain, thereby suppressing the sensation of sleep drive. However, this does not eliminate the underlying sleep debt — the adenosine continues to accumulate and the sleep drive remains, returning with full force once the caffeine wears off. Understanding sleep drive can help explain why maintaining regular sleep and wake times is important. Irregular sleep patterns disrupt the balance between sleep drive and circadian rhythm, making it harder to fall asleep and wake up at desired times.