I've looked into this a little because I work awkward shifts and I'm trying to optimise my sleeping.
Sleeping is a surprisingly active event. It's not like going into standby, which I sort of assumed it was. I had the idea that the higher functions of your brain went on standby while physical maintainence was stepped up (you heal more when asleep). In fact, there's more brain activity during some parts of sleep than when you're awake. In that respect, the OP is wrong about how much energy is used.
There are 5 distinct stages of sleeping, which can be detected by medical scanning kit, particularly brain scanning. Stages 1 and 2 are light sleep, stages 3 and 4 are deep sleep, stage 5 is REM sleep. The stages repeat in cycles that usually last between 80 and 100 minutes. The routine is 1,2,3,4,3,2,REM. That routine takes the 80 to 100 minutes in total, then it's back to stage 1 again.
If you sleep for more than 1 cycle (as you should), the total length of a cycle remains pretty much constant, but the length of stages 3, 4 and REM varies. The longer you sleep for, the more REM and less stage 3 and 4 you get.
As far as I can tell from my reading, light sleepers spend more time in stages 1 and 2, whereas heavy sleepers spend more time in stages 3 and 4. If you're woken during stages 3 and 4, you'll be in a mess. Ever had one of those days where you know you had 7 or 8 hours sleep but you're still a zombie for hours after you wake up? You were woken during stage 3 or 4. If you're woken during REM sleep, you will probably remember vivid dreams. You may also wake up paralysed, as you are always paralysed during REM sleep so that you don't move to match your dreams. It's happened to me - scared the hell out of me.
Your best bet is to make sure you sleep a whole number of cycles (the 1,2,3,4,3,2,REM cycle). How many cycles is less important. Nice idea, but without quite sophisticated monitoring equipment, you can't be sure what stage you're in. You can start with a guess of 90 minutes per sleep cycle, with 7 and a half hours being best for most people (5 cycles).
What you eat before sleeping might have an effect. I think a full stomach reduces the amount of deep sleep, which isn't a good idea. Drugs often have the same effect, particularly nicotine and alcohol. You'll be deeply unconscious with enough alcohol, but that isn't the same as good sleep.