The data during the night, which is what I care about, I've come to trust on Fitbit watches over the years. The data edit tools in the app fortunately make it easy to correct this (I'm not someone who falls asleep with the video playing, so I generally know when I turned off the activity at night and when I woke for the morning). Over the years, my Fitbit watches have fairly consistently confused idle time sitting on a couch watching videos or lying in bed reading on my phone before or after sleeping as sleep. If I'm concerned about actual total sleep times, I do generally need to edit the Fitbit data in the morning. In the daytime, as a watch, I much prefer the Garmin, but at night, as a sleep tracker, which is the main thing I want from these watches, I only trust the data from the Fitbit. I own both a Fitbit Vivo Active 4S and a Garmin VivoSmart 4. These plots definitely align with my personal anecdotal experience. But I don't know if we can draw real conclusions from that I'm also surprised they only did one night, both because you get a lot more data per amount of effort by having participants keep it for a month, and because I'd not be surprised if the software is more comparative than absolute. Or maybe your watch works differently from the ones used in this study

It could be that that's just because it's not flawless and they want people to see it's often correct even if they got unlucky the first night(s). You'd get more data (from multiple nights) without needing to recruit more participants, but you'd also be biasing towards whether the tracker works well for this set of people (probably a worthwhile trade-off, up to a point) If the software behaves no differently from one night to the next, that is, if you can take any random night's assessment and it would be as likely to be accurate as the first, then it does not matter that they had participants only wear it one night. If the software compares behavior between nights and can then say 'that was relatively good', then you're right and the limitation matters That's a number you probably can't hit if you are getting adequate exercise, quality food, and consistent deep sleep. To be fair, for sleeping poorly, I got an incredible amount of work done sitting in front of a PC 12 hours a day. Getting older it hits me how trivially simple solutions to certain problems are despite most running around in circles trying to solve them the entirety of their lives. I really wish I had understood this decades ago. If you do a combination of those things there is probably zero you can do to sleep consistently and well. Of course, all of the common sense stuff still applies: staying up all night, sleeping erratic schedules, drinking a gallon of water before going to bed, drinking a bunch of caffeine late at night, medications, stimulants, cocaine, trying to sleep in a room that's 90F+, having sleep apnea, someone drilling a jackhammer outside your bedroom window etc. Now I'm a lot less convinced that lighting stuff matters for sleeping (but I would still follow it given the the whole cancer-light link thing.) The amount of things I tried back in my 20s when it all was trivially simple: I just need to get an adequate amount of strength training and cardio exercise during the day and I sleep straight through the night.

I was considering trying a sleep tracker again last year and before that happened solved my sleep problems.

Sleep tracker says no deep sleep