I normally start to feel to feel down as a result of the shortening days as early as October. I bought this model as it is medically certified and after doing a bit of research. I feel so much better, I don't wake up feeling groggy and miserable and my whole outlook has changed. I have more energy and optimism than I can ever recall. Whether this change is due to some kind of placebo effect or is actually down to the lamp itself I don't know or care but I do know it has had a massive impact on my life.