Various hours, but it can be a rather small number of hours on most occasions. Granted, last night by the time I got to bed for some sleep I was shattered, had a good 12 or so hours sleep. Bloody deserved it too.
Can't sleep? Get out there and do a decent day's work. Get up early to prepare for the day's shift, travel for numerous hours (last week I was spending 1h45m+ each way a day to go to work) to get to the workplace, do a decent bit of hard work, spend some hours getting back home, see how bloody tired you are then. Do that all week, starting Monday and finishing Friday, see how exhausted you are then, how much you deserve a weekend off. If you've done a decent lot of work (and that does NOT include sitting in an office chair for the day! I do of course mean doing something that is more physical than typing stuff into a computer all day whilst taking several coffee/smoke breaks, with the possible risk of RSI from lifting coffee mugs all day) all week, you'll be glad to see your pay in your bank account, more so to have the weekend away from work (although your mind tends to go back to thinking about work) and doing something you like doing with your wages.
Some people are so lazy they can't sleep because they've been consuming food and drink all day, so there's a whole load of energy floating about them still when it comes to going to sleep...
Insomnia? Surprised those with it aren't claiming Incapacity Benefit...Sponging prats they are too...