It's not legally prohibited, and it's not in the guidelines.
It appears to be down to the whim of each yard/marina at present.
It's up to you to decide what "Staying Alert" means. It's no longer "Stay at Home".
If your boat in the water?
Are you thinking of setting off in the early morning? If so, I'd say it's within the realm of "driver" safety, like lorry drivers taking a kip.
Or you could always argue that you were just doing to test your eyesight. Just don't tweet about it.