For practical purposes, there are interchangeable.

However, for certain specific situations, one may be preferred, only preferred, not mandatory over the other.

還沒 is used when it is a bare, neutral statement / comment on someone not asleep yet.

還不 is used when it is questioning / accusative of why someone is not asleep yet when ought to be.

So, you have:-

很晚了，你还没睡

明天去上学你还不睡

However, no one will say you are absolutely wrong if used interchangeably.

For practical purposes, there are interchangeable.

However, for certain specific situations, one may be preferred, only preferred, not mandatory over the other.

还没 is used when it is a bare, neutral statement / comment on someone not asleep yet.

还不 is used when it is questioning / accusative of why someone is not asleep yet when ought to be.

So, you have:-

很晚了，你还没睡

明天去上学你还不睡

However, no one will say you are absolutely wrong if used interchangeably.