I use a light therapy box for treatment of SAD, and also to adjust my circadian rhythm, as I have trouble waking up at the appropriate time.
Is it important to use the box at the same time on weekends as on work days? Also, if I miss a session, should I use the light therapy box in the afternoon, or just wait until the next morning?
We highly recommend maintaining the same sleep schedule 7 days a week. Sleeping in on weekends allows the circadian clock to drift later, creating a jet-lag-type situation on Monday morning. People vulnerable to depression can feel the negative aftereffects for days after. (It's not just "Monday morning blues!") Some people feel compelled to sleep in on weekends to make for lost sleep during the work week. The solution is earlier weekday bedtime. Bedtime even 30 minutes earlier will provide 3.5 hours extra sleep per week -- equivalent to sleeping in 1.75 hours on Saturday and Sunday.
Should you miss a morning light therapy session, it is better to take it later in the day than to skip it -- but avoid the evening hours.