A 20-minute power nap is long enough to recharge but short enough to avoid grogginess. Set the timer, rest, and the alarm wakes you right on time. Keep the tab open and your volume up.
Type a number of hours, minutes and seconds — or tap a preset — and press Start. The timer counts down and sounds an alarm when it reaches zero, even if you switch tabs (the tab title shows the time left). Pause and resume anytime, or Reset to start over. Space starts/pauses; Full screen gives a big, across-the-room display.
Counting down to a date or event instead? Make a countdown. Need to time how long something takes? Use the stopwatch.