That’s down to you not having eaten for so long. You probably eat your dinner at, what, 6 or 7 o’clock in the evening? So, when you wake up, it’s been at least 12 hours since you’ve eaten. Your blood sugar levels start to drop, which makes you feel woozy and sick. Also, the acid in your stomach starts attacking your stomach lining, which makes you feel hungry and ill at the same time. All of this together can make for a pretty unpleasant start to the day 🙁
In my case its normally because I drank to many beers the night before. It is probably due to hunger. The same may hold true about thirst, which can also cause nausea. On the other hand, eating too close before bed can also cause you to experience nausea in the morning.