What causes fluctuations in deep sleep on wearables?

[TimothyXW]

I’ve been tracking my sleep with a wearable for a few months now, and I’m confused because my deep sleep duration seems to vary wildly night to night even when my routine feels consistent. I know it’s important for physical restoration, but I can’t figure out what specifically triggers a good or bad night for that particular stage. Has anyone else noticed this kind of unpredictability in their own sleep architecture?

[MichaelW]

Yeah I noticed that too. My deep sleep bars swing a lot even when I feel like my routine is steady.

[Zachary_G]

I started tracking caffeine and alcohol timing, stress levels, and meals, and the numbers still jittered from night to night.

[Victoria34]

I tried a strict wind-down ritual and a fixed wake time for two weeks, slept okay, but that restorative phase still varied a ton.

[AdamWR]

Do you think the problem is the tracker misreading things or is there some bigger factor driving the swings?