As a nervous system stimulant, caffeine in coffee, soft drinks and chocolate can make you feel as if someone pressed a "fast-forward" button on your body. While caffeine has many effects, one that is not common is causing you to feel drowsy. However, when caffeine wears off, you may notice the effects of fatigue are more pronounced. Understanding how caffeine affects your system can help you understand why you may associate drowsiness with caffeine.
When you consume caffeine, it signals the release of hormones in your body that can increase your heart rate and blood pressure. This brings more oxygenated blood to your tissues, causing you to feel more alert and awake. While caffeine can affect people differently, it should not make you feel drowsy. If you do not feel more awake, you may find you feel more jittery or anxious instead.
A Low 'Low'
Caffeine can mask your drowsiness symptoms, which can be beneficial if you are tired and need to concentrate on a task, meeting or project. However, caffeine starts to wear off gradually, and when it does, you may find you feel even more fatigued than you were before you consumed caffeine. Remember that caffeine can cover up your drowsiness, but it does not make it go away entirely. If you find you are especially sleepy a few hours after consuming caffeine, this has more to do with your lack of sleep than it does the caffeine you consumed.
A Harmful Cycle
Because caffeine contributes to alertness, it can confuse your nervous system into thinking your body does not sleep or it can affect your ability to enter a deep sleep. In this way, excess caffeine intakes can set off a harmful cycle in your body. For example, you may drink caffeine to feel more alert. However, the caffeine keeps you from going to sleep, which makes you drowsy. The circle continues when you drink more caffeine during the day to reduce feelings of drowsiness. Over time, this fatigue can manifest itself as insomnia, or an inability to fall into a deep sleep.
If your caffeine habit is interfering with your daily sleep cycle or causing you to experience bouts of energy followed by deep fatigue, it may be time to cut back on your caffeine. Because ceasing caffeine consumption altogether can cause adverse side effects, such as headache and irritability, you may wish to cut back on your caffeine intake gradually (see reference 1. Start by switching your daily cup of coffee to decaffeinated or brewing your tea for a shorter time, which reduces the amount of caffeine present.