Rest is one of the best things for a child fighting an infection, and there is no need to wake a sleeping child simply to take a temperature or give fever medicine. Fever medicine is for comfort, so if your child is sleeping peacefully, it is usually fine to let them rest.
Keep the bedroom at a comfortable temperature, dress your child in light clothing or a single layer, and avoid piling on heavy blankets, which can cause overheating. There is no need to cool the room aggressively or use cold sponging, which can make a child shiver and feel worse.
Do check on your child periodically. Wake and seek help if you notice difficulty breathing, a non-fading rash, unusual floppiness, or if they become very hard to rouse. For babies under 3 months with a fever, or any child who seems significantly unwell, contact a doctor rather than waiting until morning.