Before you fly, you will be asked to sign a medical disclaimer. This is for your safety, and the safety of your instructor. This is the relevant extract from the form:

I hereby apply for Temporary Membership of Stratford-on-Avon Gliding Club and agree to be bound by the Rules and Regulations of the Club. I declare that I do not suffer from epilepsy or from sudden attacks of fainting or giddiness or from any other disability, mental or physical, which would be likely to result in the flying of a glider by me being a source of danger to myself or to others, and I understand that it is my responsibility to inform the Club of any change occurring which would affect this declaration of fitness.

As a guideline, if you're fit to drive a car, you're fit to fly.