Sandra Rosenthal, Jack and Pamela Egan Professor of Chemistry and professor of pharmacology and chemical and biomolecular engineering, and collaborators have proposed that the seasonal rate of change in daylight has the greatest effect on illnesses with seasonal patterns, including bipolar disorder and major depressive disorder with a seasonal pattern, more commonly known as seasonal affective disorder. This means that it is the change in the amount of sunlight from the day before, not the amount of sunlight itself, that affects how a person with such an illness feels and experiences the world.