It depends on the age. Puppies sleep up to 20 hours a day, whereas adult dogs need around 14 to 15 hours. Like people, older dogs need more sleep.
Size matters, too. Big dogs sleep more than little dogs.
Researchers found that during an 8 hour night, dogs sleep for about 16 minutes, then wake up, stay awake for around 5 minutes, then go back to sleep. On average, that happens about 23 times during a normal night.
Read the science behind the article: Sleep-wake cycles and other night-time behaviours of the domestic dog. (G. J. Adams, and K. G. Johnson)
Watch this you tube compilation of dogs sleeping and dreaming.