Other people can be a great source of support and comfort to those who suffer from Seasonal Affective Disorder.
The top 5 things to do:
1. Understand the problem РYou need to recognize that SAD is a real problem and affliction.  Someone who suffers from SAD may look okay but can be having major difficulties in functioning.  Your relative or friend with SAD needs more light and can benefit from your support and understanding. Once you understand the mood and energy problems of SAD, you will be able to handle them better.
2. Just be there РJust your presence and company can do a SAD sufferer good.  Even though they may seem withdrawn and unfriendly, they appreciate the company. Many like the people being around, but you need to remember that you will probably not get a lot out of them.
3. Encourage the seasonal person РRemind them that this phase will pass, and they will feel better again.  Remind them of some good times you have had, they may simply have forgotten.  Because you understand that it is a seasonal problem, it can help just to remind them that it will pass.
4. Help with simple things РJust getting out of bed can be huge step.  But asking them what you can do to help is greatly appreciated.  It can be getting the kids ready and off to school, making breakfast or lunch or even dinner, getting groceries, household chores, doing the laundry or even paying the bills.  Some things are simple and will take a couple of minutes, but it can make a huge difference.
5. Try to understand the seasonal person when they are in the other phase Р It may come as a shock when the sufferer all of a sudden is  happy and successful.  But you need to understand that you need to be supportive either way.  Sufferers can become a bit wired from the sudden abundance of sunlight.  Many can  show  poor judgment or impulsiveness.  If you see this happening you may want to speak with your doctor.