Is Retin-A effective if only used for an hour instead of overnight?

It is most effective when used at night. I would suggest starting out with using it twice a week. Once you feel like your skin can handle that, add another day. If your skin just can't seem to handle it, try an over the counter retinol product. Revision has a 0.5% Retinol that is wonderful. Make sure you are using a spf during the day as well

Thank you for your question.  Usually, you want to taper up on RetinA use.  Try using it once or twice a week for several weeks and when able, add a extra night and build up.  Most people can't tolerate RetinA every night and will alternate days.  If you are still getting irritation and dryness, try using the prescription 0.025% version or use Retinol, the over the counter version.