Evolutionary Adaptation is the sole reason for sleep
The evolutionary theory claims that there is survival of the fittest, which means that whatever characteristics we have are there because they are or were once useful. Meddis (1979 suggested that we have evolved out sleep patterns. This means that we sleep because it enhances our chances of survival.
There is other research that backs up this idea of our sleep patterns being evolved for our benefit. For example, it has been shown that those animals that were in danger from predators did not sleep much. This was to ensure they remained aware; hence, remained alive. An example would be cattle, which have many natural predators and shrews which have high metabolic rates. These animals sleep very little. Also, cows napped (and still do) for 2 hours at a time. On the other hand, animals that can sleep safely hidden away are more likely to sleep for 14 hours. An example would be squirrels, which have safe burrows to hide in. Another example is lions which have few predators and meet their food needs in short bursts; hence, are able to sleep longer hours. These examples, suggest that sleep patterns evolve to suit the situation, and lead to the most likely chance of survival for each species.