"Yowie" wrote in
:

So, my question is: can I turn the thing off whilst asleep?

Don't do it. Two reasons:
1. Fish need oxygen, and as you have quite many fish in that tank,
theymight suffocate if the filter isn't running and moving water.
2. If a filter is shut for more than 2 hours (about), the bacteria living
in the filter media start to die or try some desperate means to survive,
and when such a filter is again turned on, it might spill poisonous stuff
to aquarium unless it is rinsed clean.

It is some kind of hang-on filter? Does the noise come from the motor, or
does it vibrate against the glass? If latter, you could perhaps put
something between the filter and aquarium.

Or then just throw that thing away and get a totally submersive filter.
They are normally very quiet. I have had one hang on filter, and hated it
all the time. And probably at least 10 inside power filters, and all but
one have been very trustworthy and quiet.

Liisa