Why is thirst not a good guide for when to drink water?

Thirst is not a reliable indicator of fluid needs because it can lag behind the body's actual need for water. By the time you feel thirsty, your body may already be slightly dehydrated. Additionally, certain factors can affect thirst perception, such as age, activity level, medications, and medical conditions. For optimal hydration, it's important to drink water regularly throughout the day, even before feeling thirsty.