While this might be helping with dream call it's not really healthy for your body. Because either way by a sleep mask or shutting curtains tight, blocking the light might damage your circadian rhythm since even when your eyelids are shut you can sense the light and that's how your body adjusts.
http://en.wikipedia.org/wiki/Circadian_rhythm
Pay attention to these parts:
Impact of light-dark cycle
Light and the biological clock
Delayed sleep phase syndrome