I remember how the pamphlets that came with my curling irons always had a warning in them along the lines of, "Do not use while asleep!" You're going to be the reason why this warning appears on the whitening strips' literature in the future! Ouch.

As the hydrogen peroxide oxidizes the colorants in the dentin layer of the tooth (in other words, as the whitening gel does its job), it incites a pulpal (tooth nerve) response. This is the sharp pain you may feel. Most whitening strips that you can take home and use are not too concentrated... probably 15%. The OTC stuff might be even less. The 35% concentration should only be used for 30 minutes but the 15% concentration instructions actually say you can sleep with them in. And even if you are awake, you can keep them in for up to 3 hours. The OP should brush their teeth to remove the remaining gel. The sensitivity *should* disappear within an hour or so.