Mild dehydration is common enough, and its effects subtle enough, that many people experience its consequences, reduced concentration, fatigue, headache, without ever connecting them back to something as simple as insufficient water intake earlier in the day. None of this is complicated in principle, yet it depends on a kind of unglamorous consistency that is genuinely easy to underestimate until you actually try to sustain it week after week.
The habit that helps most is rarely a dramatic one, keeping a water bottle visibly within reach throughout the day tends to matter more than any specific target volume, since visibility and convenience drive actual consumption far more reliably than an abstract daily goal that is easy to forget about entirely.
Building the habit around existing routines, a glass with each meal, a refill at each break, tends to work better than relying on remembering to drink water as a standalone task, since attaching it to something already happening removes the need for any separate, easily forgotten reminder.