Stimulants, such as caffeine, increase alertness and wakefulness by blocking the action of adenosine, a neurotransmitter that builds up in the brain throughout the day and promotes sleepiness. However, this process can also cause an imbalance in the brain's natural sleep-wake cycle, leading to feelings of sleepiness or exhaustion once the effects of the stimulant wear off. Additionally, stimulants can also disrupt normal sleep patterns by interfering with the timing and quality of sleep, which can contribute to feelings of sleepiness during the day.