I have a HappyLite Jr. that I use sporadically. I should use it regularly, but I don't.

It does help. It is not dramatic (at least for me), but it helps stave off the worst of the symptoms.

Use it in the morning before the sun comes up. You are trying to ADD hours of light to your day. Duration varies depending on how much extra light you need, which in turn depends on your own personal requirements and the latitude you live at and the climate there. If you live in cloudy coastal Alaska, you might need several hours per day in the depths of winter. If you live in New Mexico, not nearly so much.

Using it at night is better than nothing, but it tends to wake you up, so it would be like drinking coffee in the evening.

Try 45 minutes or so while you are getting ready for work, for example.

If you have a desk job, take it to work and use it there. That doesn't help as much, since it is already daylight, but if you have no window at work it might help a lot.

You really need to do it regularly to get good results. Its sort of like malnutrition - you can't just take a vitamin on a day when you are run down, you need it every day to stay healthy.