A sleep mask is a useful lucid dreaming aid. In fact, it can improve your sleep and dreams on several levels. If you have trouble winding down to sleep, or still feel tired even after a good night's rest, it may be because artificial light sources are interfering with your body's natural Circadian rhythms.
Why using a sleep mask?
The hormone Melatonin regulates the day-night cycle of the human body and controls the recovery phases during the sleep. For the production of this hormone, the human body requires sufficient darkness. Lightness prevents the production of Melatonin. Even a little portion of light during your sleep, for example caused by a digital clock, is a disturbance. Sleep masks however provide complete darkness for your sleep.