Arduino Starter Kit example Project 8 - Digital Hourglass

We tested the "Arduino Starter Kit example Project 8 - Digital Hourglass (Arduino 1.6.5) on 3 different Arduino Uno's in a class. When "shake" the Arduino shortly after the end of a cycle, the LED's go off and the cycle restarts but if we wait some time ( +/- > 15 minutes) nothing happens :the LED's stay on and the cycle doesn't restart. Does someone have an explication for this?