Caffeine Zone is a free iOS app that tells you the perfect time to take a coffee break to maintain an optimal amount of caffeine in your blood.
It also tells you when to stop drinking tea and coffee, so that caffeine doesn’t interrupt your sleep.
The authors of the app, doctors Frank E. Ritter and Kuo-Chuan Yeh, found that between 200 and 400 milligrams (mg) of caffeine in your bloodstream provides optimal mental alertness, and that below 100mg of caffeine is ideal for sleeping.
Caffeine Zone displays a graph of this in action (pictured above). You hit the Consume Caffeine button, pick your poison (tea/coffee/gum/other), and the app plots a graph of your caffeine level. A green bar represents the “caffeine zone” that you’re trying to hit, and a blue bar shows the level that you need to drop below before bed time. Ideally, you’ll probably drink a large cup of coffee/tea in the morning and then top it up with small cups throughout the day. One of the app’s neatest functions is that it warns you if you’re about to consume something that will leave you above the sleep threshold (in general, this means you shouldn’t drink a large cup of tea or coffee after midday). If you have built up a resistance to caffeine, or you’re particularly affected by it, you can alter the “optimal” and “sleep” values in Settings.
This is an excerpt from an ExtremeTech article, which you can read here.