Even if you can sleep in you’ll have been up all day prior
You’re assuming you have enough time off ‘back home’ to have been awake through a local day/night sleep. Maybe work with the company a little and get to the point (as I was for pretty much 20 years of my life - without the benefit of PER-LHR sectors) where I didn’t have a ‘home’ port. If you work hard enough, you won’t be awake all day before you blast off to LHR or PER. Alan could turn out to be OK. Through a delirious fog of tiredness, naturally...
And for the Angels out there thinking these comments are rubbish - just try living an aircrew pattern for even a paltry 12 months and see what it’s like. You have absolutely NO idea and if you did, you would not get on an overnight sector ever again unless you knew the entire crew had at least ten days off before you got onboard. Unlike you, I DO know what I’m talking about.