In Surveyor, Edit Route mode, hit the Edit menu which is the "page and pencil" icon, second from the left.
Choose Edit Environment, and look for the "Light Brightness" slider - the vertical one at the lower right (unlabelled of course, just to make it more obscure). It needs to be set for each time point around the clock dial, same as for the colour and fog settings. With a bit of luck, you might only need to do this once for the DHR route (or Sessions) and not adjust any of the assets they use.

Another valuable thing to know is that the Sun Colour setting also controls the darkness of the dynamic shadows. If it's set at pure black, the shadows disappear, so I recommend setting Sun Colour to a shade of grey (my preference is dark grey) then adjust the overall Light Brightness slider to suit.
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