Several Months ago, I borrowed my wife’s Fitbit to track sleep. It recorded very little Deep Sleep, something that is known to decline with age. At the time it was my habit to have one or two cups of coffee in the morning. I read Matthew Walker’s “Why We Sleep”, and he is a strong proponent of no caffeine and at least 8 hrs of sleep. ( This has been called into question, apparently approx 7 hr has lowest overall mortality, which rises as you sleep more or less). Anyway, I cut out all caffeine, used blue blockers at night and used every sleep hygiene trick I could come up with. I subsequently signed up for Whoop and started tracking sleep, 6 weeks caffeine free and then 6 weeks with my usual coffee.
Each Chart is a histogram, blue being caffeine free and red with caffeine.
My conclusion is that I can continue to enjoy my morning coffee, and that my sleep is better than that recorded by the Fitbit