Yes, indeed. I have seen the practices adopted by several depressives, all to good effect. I have seen a severe manic depressive get ironed out, first by heavy use of vitamin D supplements and then getting out in the sun and walking a lot. The D works but the sun and exercise works better for several reasons.
Another lay expert on a subject he knows nothing whatsoever about.
What you are describing is ordinary mood variation, NOT CLINICAL DEPRESSION! Many depressed patients are dysfunctional to the point they can not exercise or be left unsupervised "in the sun". Seasonal Affective Disorder (partially due to reduced exposure to certain wavelengths of light) does not account for any large proportion of cases seen in a mental health setting, particularly the rest of the year!