Cravings are naturally stronger in winter. Insulin levels are naturally higher and fat is easier to put on. Weight loss can be slowed or stalled or even reversed. In the winter an insulin surge and cravings can be caused by smells too--sweet smells, spicy smells, frying/oily smells. Cravings can even be caused by air freshener!! These things have less to do with the individual and his/her will power and determination (or lack thereof) and more to do with a natural seasonal cycle and genetic survival mechanism.

Many people are affected by seasonal changes who don't necessarily suffer full blown SAD. Read here