I don't know of any pattern since it varies all over the place from animal to animal. However, nocturnal animals like bats, cats and rodents seem to sleep a lot more hours...

However, if one uses the natural day/night cycle as a reference, then it is fair to say that it is usually dark for about 8 to 12 hours per day except for the higher lattitudes and polar regions...

Since processing energy during the day requires the 5 basic senses, when it gets dark the eye sense immediately becomes affected. Either case, the sleep cycle is directly related to how the body/brain combination are processing energy. So, for example, with human beings, most get tired when they are bored...

Also yawning seems to be contagious. Try starting to yawn and I will bet you that many others around you will also start doing it almost completely unconsciously...

Finally, I can just strat thinking about yawning and I can transmit that thought to you and many others by reading this will actually start yawning...

IMHO, a lot still needs to be learned about the sleep portion of the day cycle...

-wirelessguru1