You certainly can, for a short while. When you inflate the kayak, the materials are stretched to their capacity. Over time, if the kayak is left inflated, that material can over-stretch, weaken, or even tear. Leaving your kayak inflated for long periods of time will make it more prone to being damaged. That being said, if you want to leave your kayak inflated overnight (say, you plan to paddle first thing in the morning and don’t want to waste time pumping it up), you won’t damage it. Just make sure you don’t leave it inflated for too long.