Why do we sleep?

[Naomi]

In article <2kvobu$p9f at darkstar.ucsc.edu>,
Eric Mintz <mintz at orchid.UCSC.EDU> wrote:
>In article <2kvjn2$21v at nermal.cs.uoguelph.ca> pboily at uoguelph.ca (Patrice Boily) writes:
>>K.C. Baker (mbkxb at s-crim1.dl.ac.uk) wrote:
>>: Eric Mintz (mintz at orchid.UCSC.EDU) wrote:
>>
(deleted)
>
>Sleep is a period of inactivity, characterized by reduced metabolic rate and
>an increased threshold for arousal.  It is also defined by its electro-
>physiological characteristics - reduced level of muscle tension, increased
>slow wave activity in the EEG (for non-REM sleep).
>
>Eric
>mintz at biology.ucsc.edu
>
Can yoiu go into further detail about "increased threshold for arousal"? 
Do you refer to arousal from the sleeping state, sexual arousal, or...?
Thanks.

Kestrel