This is common in some people and depends on how much fluid shifts in your body as you sleep and how loose the skin is in the upper eye lids. See an oculoplastic specialist. You can find one at www.aao.org.
If there's nothing else wrong with your eyes (no infection or inflammation, etc), it could just be fluid collection while you're horizontal in sleep. Heredity and allergies play a role, too. You could try sleeping with your head slightly elevated, via larger pillows or a small bed wedge.