The main reasons for feeling sleepy after drinking coffee involve the effects of caffeine on adenosine, dehydration, tolerance, and sugar crashes. Caffeine blocks adenosine receptors in the brain, which normally make you feel tired. While caffeine prevents adenosine from making you sleepy initially, your body continues producing adenosine. Once the caffeine wears off, this built-up adenosine binds to receptors all at once, causing a sudden wave of sleepiness. This is often called the "caffeine crash" or adenosine rebound effect. Other reasons include:

• Developing a tolerance to caffeine, which reduces its stimulating effects.
• Coffee's diuretic effect causing dehydration, which can make you feel sluggish.
• Sweetened coffee causing a sugar crash after an initial spike in energy.

Moreover, caffeine can affect stress and sleep quality, which can indirectly make you feel more tired after drinking coffee. People who regularly consume caffeine may also experience withdrawal symptoms like drowsiness when caffeine levels drop. In summary, coffee can make you sleepy mainly due to the rebound effects of adenosine once caffeine wears off, along with dehydration and sugar crashes from some coffee preparations. Managing caffeine intake, hydration, and timing can help reduce this sleepy feeling after coffee.