Tiredness

I'm extremely confused. When I'm out I'm absolutely knackered yet as soon as I get home I seem to have bundles of energy. From about half 9 when we were out until I walked in my door 20 minutes ago I was constantly yawning, yet now I'm wide awake and will be unable to sleep for a while. It happens all the time which is completely frustrating, hopefully one day my tiredness will come when I want it and stay away when I don't.