Why are Bipolar Patients more likelly to get SAD

When you first look at the title you simply think oh they suffer from depression, so of course they are sad.  This SAD is quite different though then just being sad.  SAD stands for Seasonal Affective Disorder.  As I was going through my routine this morning and that includes any interesting topics that I find on twitter.  One tweet really caught my eye that was listed by http://www.bpkids.org/.  Per the article that Child & Adolescent Bipolar Foundation linked, stated that research shows that patients that have bipolar disorder are more likely to display symptoms of SAD.  The full article can be found at http://www.medwire-news.md/55/91425/Bipolar_Disorder/SAD_and_symptom_seasonality_common_in_BAD_patients.html