Despite the common phrase "lion's den," lions actually do not den and instead sleep on the open savanna. Lions are at the top of the food chain, so they experience no predation pressures requiring them to den. Some Egyptian lions do rest in caves during the day, but this is an adaptation to the desert, is not their usual habitat and is considered aberrant.
Lions spend a majority of their day asleep, an average of 20 hours. This is because of their size, life style and hunting style. Lions are enormous cats. They can weigh up to 400 pounds. Most of this weight is muscle. This bulk generates an enormous amount of heat, requiring frequent rest periods. However, lions need all that bulk because when they hunt they need to accelerate quickly and wrestle down their prey. Something the size of a zebra does not go down easily, especially when it is moving at top speed. Finally, lions rest as much as they do because they eat enormous quantities of food in between long periods of no food at all. Lions are not as successful at bringing down prey as hyenas, and may go a day or more without food. It is more efficient for them to rest in between meals than to waste energy in mindless activity.