The further into your pregnancy you are, the more tired you feel; it's perfectly normal! Bouts of fatigue or "hypersomnia" are caused by progesterone, a hormone produced during pregnancy. On top of that, the weight of your baby starts to take its toll on you in the last few months. At the end of your pregnancy, your sleep patterns will also be interrupted by baby moving, the size of your bump and the pressure exerted on your bladder which makes bathroom breaks more frequent.
Naps are ideal for relieving fatigue. Resting up will take some of the strain off your body, veins and spine, and will do wonders for your mood too! If you can't manage to sleep, lie down with your legs elevated to facilitate blood flow.
The best time to rest is at the start of the afternoon, when you get that "drained" feeling. Even a 20 min power-nap is enough to perk you up.