light boxes and sleep

hello i wondered of the members who use light-boxes for SAD in the winter, at what time of the evening do you need to switch it off.? I believe that they will reset the internal body-clock in the early part of the day, apparently the more light we get during the day, the better sleep should be.
I am aware that it helps with the winter blues, but don't want to find that it messes up my sleep even more.