Caffeine is considered a stimulant, meaning it causes a reaction in your body that can speed up your central nervous system, making you feel more awake and alert. While this may be a good thing if you are feeling sleepy or having trouble concentrating, an excess amount of caffeine can have an unwanted effect -- contributing to the "jitters," a nervous or anxious feeling that can keep you from staying focused and even sleeping well. Understanding how much caffeine is too much for you can help you prevent the jitters.
Caffeine can have several effects on the body and also may tend to affect people in different ways. For example, caffeine can affect some neurotransmitters that normally keep the blood vessels slightly closed. This causes your arteries to remain wider, allowing more blood to flow through them, which brings more oxygen to your brain and tissues to make you feel more awake. Because your body systems are moving slightly faster, you may feel jittery or uneasy. Caffeine also can stimulate hormones, such as adrenaline, that make your heart beat faster and cause your body to take on the fight-or-flight response you experience when you are afraid. The increase in adrenaline makes you more likely to experience the jitters.
If you are not overly sensitive to the effects of caffeine, you are less likely to experience the effects of the jitters if you consume it in moderation. A moderate caffeine intake is about 250 milligrams per day, which is roughly the equivalent of drinking three cups of coffee. However, the amount of caffeine can vary based on the type of coffee.
Use a simple test to find out if you are especially sensitive to the effects of caffeine. Start by taking your blood pressure to get a baseline measurement. Drink or eat a caffeine-containing food, such as coffee, a soda or chocolate, and wait 30 minutes, then take your blood pressure again. If your blood pressure increases between five and 10 points, you may be especially sensitive to caffeine's effects -- and more likely to experience symptoms like the jitters.
If caffeine's effects are giving you the jitters, you may be experiencing additional side effects. These include anxiety, changes in mood, heart palpitations or inability to sleep well. If your caffeine intake and jitters interfere with your daily life, you may wish to reduce your overall intake. Because your body can become accustomed to drinking caffeine, you may have to reduce your intake slowly to avoid adverse side effects such as fatigue or headache.