From Elizabeth Scott, M.S.,
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About.com Health's Disease and Condition content is reviewed by Steven Gans, MD

Caffeine is a drug, popularly consumed in coffee, tea, soft drinks and, in smaller doses, chocolate. While we seem to have a love affair with these products, there’s been quite a bit of confusion and even controversy surrounding caffeine lately. Is it good or bad for us? Here’s a brief tutorial on caffeine, and some surprising answers to these questions.

Effects on the Body:

• Hormones- You can feel the effects of caffeine in your system within a few minutes of ingesting it, and it stays on your system for many hours—it has a half-life of four to six hours in your body. While in your body, caffeine affects the following hormones:
• Adenosine- Can inhibit absorption of adenosine, which calms the body, which can make you feel alert in the short run, but can cause sleep problems later. (More on this below.)
• Adrenaline- Caffeine injects adrenaline into your system, giving you a temporary boost, but possibly making you fatigued and depressed later. If you take more caffeine to counteract these effects, you end up spending the day in an agitated state, and might find yourself jumpy and edgy by night.
• Cortisol- Can increase the body’s levels of cortisol, the “stress hormone”, which can lead to other health consequences ranging from weight gain and moodiness to heart disease and diabetes.
• Dopamine- Caffeine increases dopamine levels in your system, acting in a way similar to amphetamines, which can make you feel good after taking it, but after it wears off you can feel ‘low’. It can also lead to a physical dependence because of dopamine manipulation.

These changes caffeine makes in your physiology can have both positive and negative consequences:

• Sleep Caffeine can affect your sleep by keeping you awake longer, thereby shortening the amount of sleep you get, and giving you less time in the restorative stages of sleep, which takes a toll on your level of alertness the next day and overall health.

  Interestingly, though, caffeine doesn’t affect the stages of sleep the way other stimulants do, so it’s a better choice than speed or other ‘uppers’ to use if you need to stay awake.

• Weight Many experts believe that increased levels of cortisol lead to stronger cravings for fat and carbohydrates, and cause the body to store fat in the abdomen. (Abdominal fat carries with it greater health risks than other types of fat.) Also, if increased cortisol levels lead to stronger cravings for caffeine-laden foods, the body goes into a cycle that leads only to worse health.

  The good news, though, is that caffeine can speed up metabolism. Also, it can help the body break down fat about 30% more efficiently if consumed prior to exercise. (You must be exercising to get this benefit, though.) Additionally, caffeine can keep blood sugar levels elevated, leaving you feeling less hungry.

• Exercise If caffeine elevates levels of cortisol and other hormones for a temporary boost, after caffeine wears off, the body can feel fatigued and feelings of mild to moderate depression can set in. This can make physical activity more difficult.

  On the positive side, caffeine has been found to enhance physical performance and endurance if it isn’t overused. This, combined with its effect of fat burning during exercise, can actually enhance workouts and enable you to get in better shape if you take it at the right time.

Caffeine and Stress

Because caffeine and stress can both elevate cortisol levels, high amounts of caffeine (or stress) can lead to the negative health effects associated with prolonged elevated levels of cortisol (which you can read about here). If you ingest high levels of caffeine, you may feel your mood soar and plummet, leaving you craving more caffeine to make it soar again, causing you to lose sleep, suffer health consequences and, of course, feel more stress. However, small to moderate amounts of caffeine can lift your mood and give you a boost.

The Verdict on Caffeine

With potential negative and positive health consequences, caffeine can be your friend, but in controlled doses. Here’s what you should remember about caffeine:

• Don’t Take Too Much Because of the health risks (above) associated with higher levels of caffeine, as well as the risk of physical dependence that can come with four cups of coffee or more each day, it’s wise to limit your caffeine intake. (Withdrawal symptoms can include cravings, headache, fatigue and muscle pain.)
• No Caffeine After 2pm Because sleep is important to proper physical functioning, and caffeine can stay in your system for 8 hours or longer, you should cut off or limit your caffeine intake to the first part of the day to ensure that your sleep isn’t disrupted.
• Enjoy Caffeine With Physical Activity Caffeine is best ingested before exercise—that way your performance is enhanced and the stress-management benefits of exercise can keep you healthy and feeling less stressed throughout the day.