Ever wondered why so many people have tooth sensitivity during in-office whitening with high-powered short-wave UV light treatment but not with long-wave UV treatment?  We did, so we researched clinical trials and academic studies like the one abstracted below.  When using extremely high concentrations of Hydrogen Peroxide (we use a hybrid), the high-powered short-wave UV light didn't increase the whitening effect but it did increase tooth sensitivity.  A more balanced hybrid gel with a long-wave UV whitening accelerator used over the course of several treatments gives much better whitening without the pain.

ABSTRACT

"Objective: To evaluate the influence of light on bleaching efficacy and tooth sensitivity
during in-office vital bleaching.

Data sources: We performed a literature search using Medline, EMBASE and Cochrane
Central up to September 2011.

Conclusions: [Short-wave UV] Light increases the risk of tooth sensitivity during in-office bleaching, and light may not improve the bleaching effect when high concentrations of HP (25–35%) are employed. Therefore, dentists should use the light-activated system with great caution or avoid its use altogether. Further rigorous studies are, however, needed to explore the advantages of this light-activated system when lower concentrations of HP (15–20%) are used."
Copyright 2012 Elsevier Ltd. All rights reserved.

Source
He, Shao, Tan, Xu, Li.  (2012).  The effects of light on bleaching and tooth sensitivity during in-office vital bleaching: A systematic review and meta-analysis, Journal of Dentistry, Vol. 40, p. 644-653