The current recommendation for people in good health is up to 400 milligrams of caffeine a day. That’s roughly the amount in four eight-ounce cups of brewed coffee. Drinking that much coffee may even bring health perks, including reduced risks of heart disease, stroke, and type 2 diabetes—though we don’t know whether that’s due to the caffeine or to antioxidants or other health-promoting substances in java. (In other words, don’t expect the same benefits from caffeinated sodas or energy shots.)
But keep in mind that just because it’s safe to have that much caffeine doesn’t mean everyone should. We don’t all metabolize caffeine the same way: Some people find that even a little cup of coffee or tea can make them restless. Also, if you don’t consume caffeine regularly, you may be affected by it more intensely when you do have it. Listen to your body, and if you tend to get jittery, try spacing out your caffeinated beverages.
One more thing: If you’re a caffeine fiend, check in with yourself to make sure you’re not using a landslide of lattes to offset sleep deprivation. If you’re clocking enough sleep but still routinely feel exhausted, speak to your doctor to rule out any medical issues that can cause low energy, including a thyroid disorder.
Nutrition – Health.com