I have a Fitbit Charge HR. It's a bit glitchy at times, but within manageable levels, and it's incredibly useful. You wear it like a watch and it tracks your steps, heart rate and sleep. Steps can be useful for pacing, or at the least for getting an overview of where your activity levels have been over time. That can be useful if you want to match it up with treatments or supplements and see whether you've broadly improved or deteriorated on them. Heart rate is really useful for many of us, because it instantly shows up if you're overdoing it and should rest. It also helps you learn which activities are most draining for you. Between that and the step counter, I realised how much energy I waste pacing around when I'm on the phone, and have pretty much stopped doing that. As for the sleep monitor, it's pretty good, and it tells you the quality of your sleep in a number. I've been logging it in a spreadsheet chart, colour-coded by sleep quality as well as showing the number of hours slept, and also the sleep tracker on Medhelp is good for seeing that at a glance. I should really catch up on both of those. There's a group on the Fitbit community forum for folks with ME. Don't bother with the Flex, by the way. You have to remember to tell it every time you go to sleep and wake up, which realistically isn't going to happen, and you will really miss the HR and the more detailed data.