Coffee is a stimulant, and like most stimulants, it can make you feel more alert. But while coffee may help you feel more awake in the short term, for some people, the opposite is true; it can also make them sleepy.

There are several reasons why coffee can make someone sleepy.

One reason is because of the sugar in coffee. When a person consumes sugar, it causes a spike in their blood sugar levels. Unfortunately, this spike causes a person’s blood sugar to drop shortly after, which can make them feel tired.

The second reason is that coffee can dehydrate a person. When someone drinks coffee, they also drink caffeine. Caffeine is a known diuretic that causes a person to lose more water than usual by urinating more frequently, leading to dehydration. When a person is dehydrated, they may feel tired and sleepy. In addition, when people drink coffee late at night, the diuretic effect can wake them up in the middle of the night to go to the bathroom. This disruption in their sleep can make them feel tired the next day.

Lastly, coffee can make someone sleepy because it affects their natural sleep rhythm. The caffeine in coffee blocks the effects of adenosine, a known sleep-promoting agent. Adenosine is a chemical that helps to regulate a person’s sleep-wake cycle. When a person drinks coffee, the adenosine in the drink binds to receptors in the brain, which makes a person feel tired. Blocking adenosine disrupts the body’s natural sleep rhythm and causes a person to feel tired and sleepy.

So if you want a better night’s sleep, cut back on the amount of coffee, or switch to decaf.