Any sudden change in status or behavior should raise the question of what might be the cause. A fever or infection (even without other symptoms) could do this. An OTC (over the counter) medication, such as an antihistamine could do this (common ingredient in OTC sleep aids). There is a condition called "sundowning" that is common in moderate to advanced dementia, where early evening and nighttime restlessness and agitation may occur. Old people (in general) and dementia patients (specifically) tolerate any medication less well than younger people, and even when a medication is needed, it is likely to be at a much lower dose, so medication adjustment might be indicated. Any drugs used to treat agitation and restlessness have the potential to actually make the condition worse (use at low dose, if at all). So any sudden change in condition should tell you, something is going on.