Once an electron moves to a higher-energy orbit, it eventually wants to return to the ground state. You see atoms releasing energy as photons all the time. For example, when the heating element in a toaster turns bright red, the red color is caused by atoms excited by heat, releasing red photons. An excited electron has more energy than a relaxed electron, and just as the electron absorbed some amount of energy to reach this excited level, it can also release this energy. Anything that is alive uses energy, and so do many inanimate items such as engines and rockets .
Energy consumption generates heat.
In turn, heat causes the atoms in an object to fire off photons in the thermal-infrared spectrum. The hotter the object, the shorter the wavelength of the infrared photon it releases. An object that is very hot will even begin to emit photons in the visible spectrum, glowing red and then moving up through orange, yellow, blue and eventually white. Be sure to read How Lasers Work and How Light Works for more detailed information on light and photon emission. In night vision, thermal imaging takes advantage of this infrared emission.