I see your point, but this ignores anxiety, sleep quality and training yourself to answer your body signals.
First, targeting a metric may create a counter productive stress that can make you sleep less.
Second, the device, and the process, may induce a sleep of lesser quality.
Last, always targeting numbers doesn't force you to listen to your body, an ability that our society is losing more and more. You don't need "8h of sleep". You need to sleep at the time and duration that your feelings indicate to you. Failing to do that result in plenty of problems, including oversleeping, sleeping at bad hours, neglecting diet related to sleep, ignoring light, not adapting to punctual overloads, etc.
Now, yes, Goodhart's law is more about cheating a system. But it's kinda what you do: you try to bypass your natural way of calibrating with sleep and only see a metric. We do that for a lot of things: food, sport, work, etc. And because of that, we behave erratically.
In a good example of Goodhart's law, there's a measure that tells you something useful, but it becomes less useful when you set a target. The three points you make argue almost as much against measuring at all as they do against setting a target.
Also, your arguments center on a metric being used as a substitute for making positive changes, rather than being used together alongside positive changes. This can happen with any measure/target. For example, if you set a target of 0 cigarettes smoked per day, sure, it's possible to just stress about the target and end up smoking more. That doesn't mean the target is bad, it just means it needs to be accompanied by action to make it useful.
> And unless you live with no electricity they may fool you.
I live in Scotland - during the winter we get 8-ish hours of daylight, during the summer we get more like 17,(to the point that it doesn't actually get properly dark at night). Routine is far more important here than natural light.
First, targeting a metric may create a counter productive stress that can make you sleep less.
Second, the device, and the process, may induce a sleep of lesser quality.
Last, always targeting numbers doesn't force you to listen to your body, an ability that our society is losing more and more. You don't need "8h of sleep". You need to sleep at the time and duration that your feelings indicate to you. Failing to do that result in plenty of problems, including oversleeping, sleeping at bad hours, neglecting diet related to sleep, ignoring light, not adapting to punctual overloads, etc.
Now, yes, Goodhart's law is more about cheating a system. But it's kinda what you do: you try to bypass your natural way of calibrating with sleep and only see a metric. We do that for a lot of things: food, sport, work, etc. And because of that, we behave erratically.