If it's getting dark then yes, a light should be used. If it's daylight hours then 99% of rear lights are going to make bugger-all difference.
It is the low sun that causes the problem. There is plenty of light but it is being directed straight at you. To have a better chance of not just disappearing in to the shadows you need to have a bright light emitting light from the back of your bike.
This isn't the one I was thinking of but it serves as another less extreme example than the one that caused me to write the post.
Edit - that was 7.29 pm as my dash cam is still set to GMT. Nearly 2 hours before sunset.
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