Light. More specifically, the blue light that is emitted by the screens you’re looking at. It has a physiological impact, delaying the release of melatonin (the sleep hormone) for up to an hour. Our bodies use many cues from the environment to regulate the circadian rhythm, one of which is the wavelength of light - which we confuse with artificial light sources. Avoid screens and blue coloured lights for a little while before bed and you’ll find sleep comes easier.