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.

The diet for longevity
Certain foods can provide the bulk of the nutrients the body needs as it ages. Here are some of the best resources.