Why is alcohol purged from the body more slowly when we sleep?
You are right, Culprit is Ethanol in Alcohol. It contains 2 carbon atoms, 6 Hydrogen atoms and 1 Oxygen atom.

Ethanol is readily soluble in water, so it easily dissolves in the
bloodstream and gets carried to various parts of the body. The most
affected areas of the body include the liver and the brain. In the
brain, ethanol adversely affects Glutamate receptors, which causes
slurred speech, memory loss, slow reaction to stimuli, and other
common side effects of being a bit too tipsy.
Not only this, ethanol molecules bind with GABA (gamma-aminobutyric acid) receptors, which is able to slow down the
activity of brain. Unlike glutamate receptors, ethanol actually makes
GABA receptors more receptive, causing the brain to slow down even
further. However, alcohol doesn’t just act to slow everything down in
the body. It also stimulates the production of dopamine and
endorphins, chemicals that produce feelings of pleasure. This is the
reason why people choose to get that alcohol “high”, despite knowing
the negative consequences that it may lead to.

So basically when you are physically working body needs more oxygen while Ethanol only has one oxygen atom, and Since Liver starts to work on regular pace, Which plays a central role in all metabolic processes in the body.
Everyone's oxygen levels in the blood are lower during sleep , in fact while sleeping or in meditation body needs very less oxygen to survive.

Why would I need to wear oxygen while sleeping?
Everyone's oxygen levels in the blood are lower during sleep, due to a
mildly reduced level of breathing. Also, some alveoli drop out of use
during sleep.
If your waking oxygen saturation is greater than about 94 percent on
room air, it is unlikely that your saturation during sleep will fall
below 88 percent. However, your doctor can order an overnight oximetry
test if there is a question about your oxygen saturation levels while
you are sleeping.

In general, metabolic rate is higher when you are awake, because then you tend to be moving about doing stuff.