Every morning when you get up, there are probably a few things you do like clockwork: pee, brush your teeth, pour your morning cup of coffee, etc. You also probably have an erection.
Morning wood, a.k.a. the boner you get the first thing you wake up in the morning, a.k.a. nocturnal penile tumescence (say that 10 times fast!), is extremely common. In fact, if you don't get morning wood, it could be a sign that there's something wrong with the plumbing down there.