Light is definitely helpful for depression. However, it also is helpful for keeping you awake and alert.
For people who have poor sleep but also may have Parkinson's disease or diabetes, light therapy can help with both conditions. In an amazing article, courtesy of Healthline, it even gives multiple referenced studies why bright light exposure has been helpful with seasonal affective disorder, workplace fatigue, and even sleep.
What you are essentially doing during the day with light therapy is telling your body that it's time to be awake and move.
Even when it is not the winter I use a SAD lamp year-round to blast light into my eyes. It's helped me feel awake and alert. I don't use it near bedtime because studies have shown it can mess up your Circadian rhythm (which is why TV and screen-time before sleeping is terrible).
This is a similar lamp to what I've been using for years.