Make iMac Automatically go into sleep at startup ?

I am looking for a small program that will put my iMac into sleep mode immediately after starting up.
Probably have it as a login item.
I like to switch on my iMac, then go and do something, like make a cup of tea, have a shower etc, and would like for it to auto go into sleep mode immediately without my input, to save screen, HDD etc

I've googled to no avail, please help.
Perhaps even a simple terminal command that the iMac can auto run.
Or something that emulates the keyboard shortcut for sleep, which I use a lot.
I don't want to set Energy saver to zero.

Why not make a cup of tea, have a shower etc and then turn your Mac on?

John T has the most obvious solution .......

But if you have to do it just set your energy saver preferences right down. It'll sleep after 1 minute.

You can always use a tool like caffeine to keep it awake when in use.