When one is thirsty and feels the need to drink, it is already a sign of dehydration.
Fatigue may also depend on a dehydration condition.
The body has a constant need for water, whether doing sedentary or physical work. Getting used to drinking continuously keeps the body well hydrated and keeps fatigue away.
Memory loss from drugs
Tranquilizers, prescribed to reduce anxiety and agitation, or even drugs such as sleeping pills can act on memory by reducing it, but they are not the only ones since antihypertensive