Have a look at some impressive matter before going to sleep (4,2,3)

'before going to sleep' is the definition.
I can't tell whether this defines the answer.

'have a look at some impressive matter' is the wordplay.
I cannot quite see how this works, but
'have' could be 'read' (reading is a kind of having) and 'read' is present in the answer.
'at' could be 'in' (synonymous in some cases - eg at school, in school) and 'in' is located in the answer.
'matter' could be 'be' (Be is an example) and 'be' is present in the answer.
A single letter 'd' remains which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?