Turn it up...?

Is your preamp/AV receiver set to night mode? That could explain. 

I’m thinking it’s something else. I know my hearing is not the same when I get up in the morning as it is in the afternoon/evening.

All the best.

JD

@mapman     Agree.  Need to measure sound levels before jumping to subjective conclusions.

@imhififan      Unlikely voltage variations can cause this.  In UK +-6% variation is the legal maximum.  Most amps draw power and store it.  Once there is sufficient in store the amp will perform consistently.

Hmmm...my space runs 'bout 24 dB typically...quiet enough that the calibrated mic on the RTA picks up me clicking keys....(5ish' ft., 95 deg. out).

Cell ap, fairly popular...

Daytime is subject to 'the world outside'....*L*

Typically the noice in power is much lower at night… so, the distortion and high frequency noise is lower. That can sound like lower volume… blacker background. 

Are you measuring with a meter?  It is merely conjecture otherwise.  Good thread starting material......

Much less ambient noise at night so you don't need as much volume to hear.

Check your mains voltage.

Someone sneaks in and turns the volume down at night with the remote so you don't notice.. 

They are trying to sleep.