SSB is always more efficient than A.M.  More of the power goes in to
intelligible information.
As night advances, the ionosphere becomes thinner, and the reflective layer
rises.
This means that signals travel farther around the earth, and the MUF maximum
usable frequency lowers.
So signals on the higher frequencies tend not to return to earth.
When the sun rises, the process is reversed.
So near sunset, you may not hear signals east of you, but ten meters may be
full of signals from Japan and the Western Pacific.
A.M. is the same mode used by A.M. broadcast stations.  It can be detected
on the most basic radios.
What A.M. tends to lead to, is more lengthy relaxed style of conversation,
often by people who enjoy creating the best asounding audio that they can.
Lots of hams who are broadcast engineers, and technically very well
educated.
Tune around and when you hear a carrier with sound, change to A.M. and enjoy
ham radio the way it used to be.
Now, I might be slightly off here, but here is my understanding of it.
Let's say you are running a 100 watt transmitter.
On sideband, your energy is changed in to one side of the signal, the peeks
in the audio are measured, and then the effective power is reduced because
the amplifier has to be less efficient to eliminate the audio distortion
that comes with higher effeciency of amplification.
So you get 100  watts times 22 divided by 2 for a peek output of 100 watts.
50 watts real output.

Now, we have a 100 watt A.M. transmitter.
The carrier is running 100 watts.  It can carry 50 watts of audio before
distorting.  This audio has two sidebands, so the real audio power is 25
watts.
I have been operating and enjoying A.M. for 48 years.  Everybody sounds on
frequency, but if you were listening in the sideband mode, you would rapidly
find that this is not the case.

Yes I know that the carrier with two sidebands is modulated to 400 watts of
peek power, but this doesn't help much.