Depression isn't a little switch you turn on and off, at least not clinical depression which is what I presume the study is talking about. You may mean the colloquial-used "depression" aka situational/reactive depression that many use to mean "really sad for a while" but it's not the same.

See this article from UC Berkeley for an explainer. Here are some quotes:

> Clinical depression is a serious medical illness that negatively affects how you feel, the way you think and how you act. Individuals with clinical depression are unable to function as they used to. Often they have lost interest in activities that were once enjoyable to them, and feel sad and hopeless for extended periods of time. Clinical depression is not the same as feeling sad or depressed for a few days and then feeling better. It can affect your body, mood, thoughts, and behavior. It can change your eating habits, how you feel and think, your ability to work and study, and how you interact with people.

> Clinically depressed people cannot "pull themselves together" and get better. In fact, clinical depression often interferes with a person's ability or wish to get help. Clinical depression is a serious illness that lasts for weeks, months and sometimes years. It may even influence someone to contemplate or attempt suicide.

> Feeling sad and depressed is often a normal reaction to a stressful life situation. For example, it is normal to feel down after a major disappointment, or to have trouble sleeping or eating after a difficult relationship break-up. Usually, within a few days, perhaps after talking to a friend, we start to feel like ourselves again. Clinical depression is very different. It involves a noticeable change in functioning that persists for two weeks or longer.