In my opinion, laser or light activated whitening is not a good treatment. The only good thing about it is that in some cases, you will have whiter teeth right away. However, a lot of the whitening effect is due to dehydration of the enamel (from the 2 hours your teeth are isolated). Once the teeth re-hydrate with saliva after 24 hrs, some of the effect will go away. The teeth may stay ligher for a while but with- in 6 months, you'll be back to where you started. Also, the gel used is very strong (up to 35%) and can cause sensitivity and leave the teeth prone sucking up stains for a couple of days. If you did not consume anything with color, and keep your teeth clean, then maybe you used a rinse, or smoked.
It is much better and cheaper to whiten your teeth at home with custom made trays. You can control it better and achieve a much lighter shade by a gradual daily whitening. Also, after initial whitening for 1-2 weeks, you can then touch-up for 3-2 nights to maintain it.
Research has shown that the laser or light does not enhance the whitening. The whitening was the same with or without the light. It's the gel that does the trick.
Hope this helps
Dr. T