TDK
My standby starts at 4Z, do I:
1. call now and declare myself unfit,
2. wait and see if they call me and then tell them I'm unfit,
3. report if called and rely on caffeine and my collegue to get me through 4 sectors because sick leave (and unpaid dependancy leave as my manager will say this is) is part of the matrix my company say they will use for redundancy selection which they frequently refer to in various subtle and not so subtle ways. I've already had a week of unpaid dependancy leave this year in addition to two weeks unpaid paternity leave to support my partner.
You should be able to take that decision your self, if not maybe piloting is not your call.
Your private "setup" should never be aloud to interfere with other peoples safety.
If you are professional you know if you are fit to fly, period!