Can seasonal depression lead to depression later on?

Hello and thank you for posting your question in this forum. We will try to unpack some of this and hopefully provide some clarity.

Depressed mood usually arrives in periods of time which can last from two weeks to a period of many months in events known as major depressive episodes. Episodes are given a minimum length of two weeks, in our diagnostic criteria, but can last many months beyond that point. There is not really a maximum length of time given in the criteria, thus your depression can be markedly longer lasting with each passing episode. In that experience we expect to see your depressed mood last most of the day, nearly every day, with you experiencing diminished interest or pleasure in all or almost all activities (even your favored hobbies). We also would be able to notice significant weight loss, insomnia, a psychomotor slowing (not being able to process information as quickly or efficiently), fatigue, feelings of worthlessness, and diminished ability to concentrate. It's important to note that we might not see evidence of all the above but if we see evidence of 5 or more of the aforementioned symptoms we would conclude you are experiencing a major depressive episode. 

Now that we have established that, let's think about some of your other questions. To understand why you might be experiencing these episodes we might analyze your life at the moment. Often these sorts of episodes can come about on the heels of important losses, grief, and other tragic events. To answer your next question on how they can be prevented: we treat them by being proactive. This again means you might need to enter therapy. Long term depression, when untreated, can lead to a total loss of interest in life. As one example, people may end up sleeping all day if these symptoms are not treated. Your life is important so it is important for you to seek care, not necessarily here, to treat depressive episodes. Ultimately, we hope this is what you will do. As a precautionary measure, if you are (or have been) feeling suicidal you can reach out to the free National Suicide Prevention Hotline at: 1-800-273-8255. Please take advantage of that free tool, if necessary. 

Thank you again for posting your question in this forum and I wish you good mental and physical health, in the future.