I wake up pretty early in the morning and spent lot's of time indoors, but in a fairly bright room. Around 15 pm I usually walk to the gym for 30 mins, so I get quite some sun exposure
My issue is that: I can apply sunscreen only once per day, so when would you recommend to apply it? In the morning or sometime before I go to the gym?
Unless you sit right next to a window in the morning, I’d say it’s better to apply a sunscreen closer to your gym walk, for example at around 1 pm. I would still protect you well during the walk, and you’d get a few additional hours inside when the sun is the brightest.