I often drink the leftover water in my bottle on the very next day, is it safe to do so?

Your question is befitting for the present lifestyle!
Water left in a bottle for a long time is unsafe for drinking, because when our lip touches the rim of the bottle while drinking, the dead skin, dust, and sweat covering our skin get backwashed in the remaining water. Our saliva also carries a lot of bacteria and can also be mixed with the water in our bottle!! Especially when it is kept to rest over a night, bacterias will grow and can enter your body if you drink the same water again. It can cause numerous health-related issues. The situation can be even worse if it is shared with someone suffering from a communicable disease.