  I propose that the need for sleep and the occurrence of dreams are intimately
linked to the physical processes underlying the continuing replacement of cells
and renewal of biomolecules during the lives of higher organisms. Since one
major function of our brains is to enable us to react to attack, there must be
truly compelling reasons for us to take them off-line for extended periods, as
during sleep. I suggest that most replacements occur during sleep. Dreams are
suggested as part of the means by which the new neural circuitry (I use the
word in an informal sense) is checked.
