OK - I have learned anew what I used to know (age does get in the way of progress. Thanks all for the comments, particularly the reboot suggestion. I rebooted the printer (turned it off, waited one minute, turned it back on) and now the sleep mode is just as advertised.
The 5000 seems to have a variety of strange behaviors that can be fixed by this power off, wait a minute, power back on reboot. Maybe fragile firmware, maybe buggy chip set in the printer - but the fix is painless enough. Probably should be the first thing to try when strange behavior is evidenced. I used to know this stuff - back in the days of Osborne PC's and CPM OS's.
Thanks to all for the responses.