An Idea for a Noiseless Wake Up Call

You don't need a beeping cell phone or a ear-splitting alarm to jolt you from your stupor. Waking up on the dot is possible without the clamour of a clock if you just use your thought power. All you have to do is to ensure that the last thought before fall asleep is: "I wish to wake up at X AM or PM". Your biological clock will carry out this order without fail.