Expose yourself to full spectrum light.

During the winter months, it can be challenging to get enough natural light. At the same time, not everyone has the opportunity to expose themselves to daylight even in summer. A good alternative is a full spectrum lamp or light therapy lamp. You can simply place it on your desk and use it while you work or study, preferably for 30-90 minutes for full effect. 

Why it helps:
Light therapy is often used to treat seasonal affective disorder (SAD). It also helps with other mood disorders and sleep problems linked to circadian rhythms. It provides almost the same effect as direct sunlight and helps to restore the circadian rhythm.