>We are very good at describing what happens when we sleep and what goes wrong when we don't, and so there are a number of theories as to why we sleep I'm not

>We are very good at describing what happens when we sleep and what goes wrong when we don't, and so there are a number of theories as to why we sleep I'm not sure I understand the distinction you're trying to make. There's not purpose in a volitional sense in evolution, only effects. We sleep because necessary functions occur when we do and problems happen when we don't.