It's probably cause you spend a lot of time in the sun without sun screen. Use an exfoliating cream and sunscreen.
What can I do for my skin? It becomes darker day by day?
In my opinion, firstly, you can adjust your bedtime, because sleep is an very important factor for skin protection. And then, you should develop reasonable dietary habits. If these ways don't work, I think you could try some whitening products.
Hope this is useful.
