Myth or fact? (142/365)

My mum always fills up a cup of water and place it beside my bed before she goes to sleep. She says cos we sleep on aircon room, by doing so, it prevents the aircon from sucking away out moisture. Now, is that a myth or it really works?

I am not too sure myself because I put moisturizer before I sleep to prevent my skin from drying. But since she still insist on the water beside my bed, it could possible give me double protection. Why not?