I'd guess, like another commenter, that some people might get this type of oscillating behaviour due to feedback mechanisms: working for 48 hours straight (enabled by mania) could trigger a recession, requiring intensive recovery, which enables a new visible episode of mania.

Note that the "off time" need not be matched by a depressive episode (and in fact I find that unlikely, since depression is not very restful) -- it could simply be exhaustion hiding the symptoms of the underlying (long) manic episode.