Then comes sleep, which, like everything else in space, requires adjustment. In space, there is no clear distinction between up and down as on Earth. Any surface can serve as a floor, wall, or ceiling, allowing astronauts to sleep anywhere. However, weak gravity means that astronauts may slowly float away while sleeping. To prevent this, most astronauts on the ISS sleep by securing their bodies in small sleeping compartments or sleeping bags. Also, due to the large amount of equipment on the ISS, the sound of air-conditioning fans and machinery is always present. Astronauts disturbed by these ambient noises and who have trouble sleeping may use eye masks and earplugs.