can deter this adaptation due to the body needing to adapt to more stimuli than just a simple sound alert. Alarm signals for impaired hearing The deaf and hard of hearing are often unable to perceive auditory alarms when asleep. They may use specialized alarms, including alarms with flashing lights instead of or in addition to noise. Alarms which can connect to vibrating devices (small ones inserted into pillows, or larger ones placed under bedposts to shake the bed) also exist. Time switches Time switches can be used to turn on anything that will awaken a sleeper, and can therefore be used as alarms. Lights, bells, and radio and TV sets can easily be used. More elaborate devices have also been used, such as machines that automatically prepare tea or coffee. A sound is produced when the drink is ready, so the sleeper awakes to find the freshly brewed drink waiting. See also Delayed sleep phase syndrome Digital clock Knocker-up Light therapy Teasmade Timer Wake-up call References Sources External links American inventions Alarms Clock designs Sleep