I think the advice to drink 8 glasses of water a day is because some folk don't drink enough to flush their system.I think it should be a case of a person experimenting with what suits them best and then trying to remember to drink water rather than sugar and additive enriched liquids all the time.Coke is fine in moderation but not all the time.
I do know that the more water I can drink before I go to bed the better nights sleep I get and I'm less likely to be heavy headed the next day.
Incog.