Why do I get a headache whenever I step out in the sun? Eyesight is normal but still!

If you feel headaches only when you are under the sun, they might be 'heat-induced headaches', most probably they aren't due to any underlying disease. It may occur if you are dehydrated. Dehydration can trigger both a headache and a migraine. It can last for up to 3 hours.

You can avoid such situations if you,

Care to stay hydrated
Use Sunglasses
Ta...

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 ...