The primary function of a sleep mask is to block out the light, making them handy for those that require total darkness to sleep (while their bed partner keeps the light on to read, perhaps, in summer when it’s light late into the evening, or while snoozing during the day before a night shift). They’re also great for traveling when you never know where you might need to catch a few Zs. And, for the beauty queens, masks with deeply recessed eye cups are great for protecting eyelash extensions while napping before an important event.