A bunch of stuff comes to mind:
- Sun and vit D (already mentioned)
- Our immune system changes during the summer, see
https://www.livescience.com/50806-seasonal-variation-human-genes-immune-inflammation.html- General feeling of wellbeing (caused by sunlight and warmer temperatures)
- Stress might be an issue. During summer, people often work less, and go on a holiday. Also colleagues and customers might go on holiday, which may cause a reduction of stress.
- Exercise. During summer, people might do more exercise (e.g. take a bicycle to work, or go surfing, swimming, diving, etc.)
- Socializing. During summer, people might be more social, which may affect mental symptoms.
- Cold showers. People might take more cold showers during summer. Some people indicated some relief from cold showers.
Some things that might be related but I don't know how:
- During summer, people sweat more, and lose electrolytes and sodium.
- Because of the heat, the scrotum hangs lower (again, not sure what the significance would be for POIS, but should be considered).
- Circadian clock is affected by seasons.
If you want to experiment, you could try:
- Light therapy lamp during winter, e.g.
https://www.usa.philips.com/c-p/HF3418_60/energylight- Saunas and/or cold showers during winter