We're all discussing effects of Martian sunlight exposure on structures, plants, solar panels, but I don't think I've seen any discussion of its effect on humans. There's a number of effects of sunlight on health, both adverse and beneficial. Some can be fought with supplements, like Vitamin D deficiency, others not so much.
Seasonal affective disorder (SAD) is a mood disorder subset in which people who have normal mental health throughout most of the year exhibit depressive symptoms at the same time each year, most commonly in the winter. People may sleep too much or have little energy. The condition in the summer can include heightened anxiety. [Wikipedia].
And suffering from this, I can say it can be quite crippling. Artificial light really doesn't cut it. In difficult, dangerous environment like Mars, this could spell doom of the colony as people simply fail to find the energy to perform the necessary tasks. And as I understand it, the amount of sunlight is comparable to deep shade or a cloudy day at best, and the access to natural sunlight through windows/skylights will be restricted due to risk of exposure to ionizing radiation (coming with solar wind and CME). And it won't be seasonal, but permanent.
Are there plans, experiments, solutions that address this concern? "Just add more LEDs" may seem as the simple solution, but will it be enough?