Well, Tamagotchis sleep over 10 hours. There's nothing to worry about it.
If it sleeps on day, then you should check your Tamagotchis clock if it's set incorrectly. Tamagotchis should go to sleep 8 PM, 9 PM, 10 PM or 10.15 PM, and wake up at 8 AM, 9 AM or 10 AM depending on the character you have.