Ultrahuman Caffeine Window and timing explained

What does Caffeine Window do?

• A daily green window. The stretch when caffeine does the most for your focus and the least to your sleep. It opens about 90 minutes after you wake, once the natural alertness lift of waking has faded.
• A personal cut-off. The point after which a fresh coffee would still be working at bedtime. It's timed to when you actually sleep, so it moves with your schedule rather than sitting at a fixed hour.
• A running estimate of what's left. Log a drink and the app tracks how much caffeine is still circulating, so you can see where you stand before deciding on the next cup.
• Adjustment for how you slept. A strong night may earn you a slightly later cut-off. A poor one may bring it earlier, or prompt a suggested Caffeine Detox day.

Why caffeine timing matters

While you're awake, a sleepiness signal called adenosine builds up in your brain. The longer you're up, the stronger it gets, and it's one of the main things that eventually sends you to bed.

Caffeine works by blocking that signal, so your brain stops registering how tired you are. The tiredness is still there; caffeine only masks it, which is why your energy drops sharply once it wears off.

Caffeine also leaves slowly. About half of a cup is still in your system five hours later, which is why a mid-afternoon cup can still be working at midnight. Five hours is only an average, though. For some people it's closer to 1.5 hours, for others nearer 9.5, which is why a single fixed cut-off suits nobody perfectly.

How the Ring builds your Caffeine Window

The Ring has no sensor that reads caffeine in your blood, and no wearable does. Your Caffeine Window is built from two things it can work with.

Your own sleep: The Ultrahuman Ring tracks when you fall asleep and wake, so it places your window around your real schedule rather than the clock on the wall. Wake at 6am and your cut-off lands earlier. Work night shifts and the whole thing moves with you.

The drinks you log: When you log a coffee or tea, the app estimates how much caffeine it contained, then tracks how much is still circulating as the day goes on. That's a well-informed guide based on averages, not a lab reading.

Together, those give you a personal cut-off: the time after which a fresh coffee would likely still be working against your sleep at bedtime.

What the research says about caffeine and sleep

Six hours before bed is still too late. The classic demonstration gave participants 400 mg of caffeine (about two to three cups of coffee) a full six hours before bed. It cut measured sleep by more than an hour, and the participants didn't notice. Feeling fine is not proof that caffeine left your sleep alone.

Across the evidence, the number is closer to nine hours. A review of 24 studies found caffeine shortened total sleep by about 45 minutes and trimmed deep sleep in particular. To protect total sleep time, the authors advised finishing a standard cup at least 8.8 hours before bed.

Dose matters as much as timing. A crossover trial in 23 healthy young men compared two doses taken four hours before bed. The 400 mg dose cut total sleep by around 51 minutes and deep sleep by roughly 30 minutes. A 100 mg dose at the same four-hour mark had no measurable effect.

So a single small afternoon coffee is a different proposition from a large pre-workout drink, and a useful cut-off has to account for both how much and how late.

How to use Caffeine Window in the app

Caffeine Window sits in the Windows dial, alongside your Circadian window. On iOS, and soon on Android, you can pin it to your home screen as a widget.

• The green zone is your window, when caffeine is most effective and least likely to touch your sleep.
• The early yellow stretch is the wake-up buffer, the roughly 90 minutes after rising when the app suggests holding off.
• The red zone is after your cut-off, when a fresh coffee would likely still be active at bedtime.

You log drinks from the Caffeine Bar, a searchable library of coffees, teas and energy drinks with their caffeine amounts built in. Each entry updates your estimated body caffeine, so you can see how much is still working through you before deciding on another cup.

Caffeine Window also reads your other Ring data. Through Brain Waste Clearance, a strong night may earn you a slightly later cut-off, while a poor one may bring it earlier or prompt a suggested Caffeine Detox day. Through Circadian Alignment, the window tracks your body clock, so an irregular or shifted schedule moves it with you.