The sides of the spectacles should be adjusted to the shape of the face so that no pressure can be felt on the temples. Often, such pressing at the temples is misunderstood to mean that the spectacles are sitting properly. Optimally fitted sides should only cause a slight pressure behind the ear to keep the weight of the spectacles from bearing down on the nose. However, if the spectacles are too tight, the sides "wander" up the sides of the head and the spectacle frames slide down the nose.
By the way, in certain situations the sides of plastic spectacle frames can deform slightly due to changes in temperature caused by the weather. If this causes discomfort, your optician can get the spectacles back to their original shape easily.
Tips for active sports fans: Silicone tips and covers for metal spectacle frames are water-repellent and ensure a tight grip and optimum fit even during heavy sweating.