Long haul flight - can i sleep?!

hi! I'm flying back from new zealand to the UK tomorrow. The first leg is 12 hours and the second is 15. I'm conscious the advice is to walk around every 30 mins - an hour but I'll definitely want to sleep for a longer period, otherwise i'll feel wiped (and the time will go super slow lol)

Does anyone have any similiar experiences or advice to share? Tysm x