burning the candle at both ends

Just sat watching the BBC news this morning and heard the item on the effects of lack of sleep. Must admit that I have recently been working late nights and early mornings (called burning the candle at both ends) especially with the unpaid work I have been doing outside of my regular employment.
Anyway, last night I went to bed earlier and clocked up a good 7 hours sleep, and must say I felt much better this morning as a result.
I notice in my reading on the Agile development methodology (philosophy/approach/paradigm?) that it is stated the team should not work more than 40 hours per week. And I can see why. Performance suffers when working longer hours.