They burn the candle at both ends.
The idiom was translated from French in 1611. It can be found in
Randal Cotgrave’s dictionary. Initially the idiom meant that one was
dissipating one’s wealth. Meaning that you were causing your money to
disappear. This is because candles were expensive and burning them
cost money. If you were burning the candle at both ends you were being
wasteful. You would also have to hold the candle horizontally, leading
to the wax dripping off and being wasted.
The idiom has since taken on a different meaning. Both ends refer to
the end and beginning of the day. If you have to light a candle then
we assume it is dark outside. Thus, if you burn it in the morning you
are up before the sun and if you burn it at night it is already dark
when you finish.