Yes, I'm serious. As to the whys and wherefores, I have no idea.
Those who know me know my thoughts on it. I would imagine it's a combination of long periods of time off together, long periods at work together and shift work (easier to lie to 'other half'....'yes dear, I've got a night shift tonight!').
Personally I don't have a problem with the sleep patterns, but then I can get by on little sleep. Many of my colleagues have problems sleeping before an early morning shift.
Regarding illness/tiredness 'affecting my work', the simple answer is that you shouldn't be there if that's the case. Take the day off.