There are many reasons to cause dry eyes such as decreased tear production, excessive tear evaporation, and an abnormality in the production of mucus or lipids normally found in the tear layer, or a combination of these. Some medications, such as antihistamines, antidepressants, beta-blockers, and oral contraceptives, may decrease tear production and then results in dry eyes. And if blinking is decreased or if the eyelids cannot be closed, the eyes may dry out because of tear evaporation. So if you spend much time reading, watching TV, or performing a task that requires close attention with the eyes, you may not blink as often, and this decreased blinking allows excessive evaporation of the tears, then you will feel dry eyes when you sleep. Some other certain conditions, such as stroke or Bell's palsy, make it difficult to close the eyes, as a result, the eyes may become dry from tear evaporation. Check which is the reason of your dry eye when you sleep.