Yeah, I feel yukky too. We usually get the 11.15am Virgin flight from Gatwick which means getting up at 3.30am, setting off shortly before 5am to check-in at 7am (I'm a nervous flyer and I like to get settled). By the time we've cleared immigration and customs at the other end, collected the hire car, done a shop at Publix en-route and arrived at the villa, it's usually about 6.30pm local time (so 11.30pm as far as our bodies are concerned). I guess it's not really surprising that we feel so tired having been up for 20 hours. After a shower, a dip in the jacuzzi with a glass of wine and a good nights sleep, we're up at the crack of dawn and raring to go.
Coming home is a whole different matter. We're normally up early on the last day to get packed and out of the villa. The last few times we've been on the late flight which leaves MCO at around 8.35pm EST. I don't sleep on the plane (I really wish I could) and by the time we've driven home, we've been awake for about 24 hours. I always feel much worse than I do on the way out. I think it's a combination of the night flight, feeling more nervous because there's nothing to look forward to (not logical, but definitely a factor), and feeling as though you've arrived home a day and a half after you got up because we're 5 hours ahead. We always used to go straight to bed as soon as we got home, but the last few times we haven't felt the need, although I'm not sure why. We managed to stay up until around 9.30pm last time.
Following some threads on these boards, I did buy some Melatonin a few trips back. It's not available here, but it's sold as a food supplement in the States. I do use it occasionally when I can't sleep and it does work (at least the 3mg dose does). However, I'm reluctant to take it on the plane because I already take a cocktail of Diazepam and alcohol and I think it could be one muscle relaxant too many! I'd be a blancmange!