My brother have problem of dry mouth and bitterness during sleep at night , because that he got broken his sleep at night ?

Mouth Breathing: Some patients with sleep apnea who snore or breath through there mouth will dry their mouths out at night. A hydrated mouth is less susceptible to dental decay because saliva is an acidic buffer. Consult a dentist about some appliances he might wear at night like a snore guard. A dentist can also recommend some products that may help keep him hydrated overnight like biotene.

Dry mouth can be: The result of many causes - due to medical condition, medications etc. Best to be evaluated by a physician. In the meanwhile there are over the counter products like biotene than can offer some relief. Hope this helps.