I notice that as the day draws to a close and natural light reduces the images on my iMac become darker. I have read that turning off “automatically adjust brightness” and also possibly “truetone” will alleviate this. I have yet to try but wonder if this is true or are there downsides?
Thank you
I try to keep the incoming daylight at pleasant levels and never use any form of automatic brightness adjustment.
Had to look for “truetone” … ok, that’s with Apple stuff. So, no idea if / how that works.
Turn off TrueTone.
Turn off automatic brightness and use something like Calibrite to set your brightness to 80Cd/M². (At least this is good for print matching) and calibrate the screen.
Don’t work with the screen in bright light.
Thank you folks - giving it a spin now 
Yes, disable any convenience brightness or ‘blue light’ compensation your monitor or computer / os offers to do for you. You want to be viewing an unadulterated image that accurately reflects your work.
Proper monitor calibration is a huge and often overlooked part of workflow.
And if you are working in a room or office with a LOT of ‘natural light’, consider investing in a set of light shields / hoods or blinders for the sides and top edge of your monitor.
I regularly check calibrate my monitor using a Spyder X and have a monitor shield mounted on it, hopefully to give some consistency.
Another reason not to reduce the brightness of the display: As brightness decreases, there are perceived differences on color. For example, reds appear much darker.
Unless you are print matching, when it is imperative that the emitted screen brightness is as close as possible to the reflected paper brightness.
It’s called the Purkinji Shift; here’s more information. What Is the Purkinje Shift and How Does It Work? - Biology Insights