Seka's right, when I was pregnant I read that it's worth using your legs, so I would get up and wander around the house, go to the loo, get a drink, taking the long way each time, and then try to sleep. A bit of exercise helps.
Definitely worth checking out the magnesium though - I can't remember if I read that sports drinks with electrolytes help...maybe that's for cramps...your midwife/ob might have some ideas too, if what we've suggested doesn't work.