Can Alzheimer's medications cause nightmares and irritability?

My father, who is 82, is in the moderate stage of Alzheimers. He has been taking Namenda 10mg twice a day and Aricept 23mg at bedtime. Lately he has been more and more irritable and has been having horrible nightmares that wake him up yelling and jumping out of bed. Do you think this could be because of the medication? I have heard that Namenda can cause increased irritability after a while. Is this true, too? Thank you.