We have all heard or read stories, especially in the old days of a trainer hiding a horse in the dark, all you hear is hoof beats and a horse flies by you.
The myths of horses working in the dark is, indeed, a myth. If someone wants to tell you that saw a horse work in the dark and he was absolutely flying, they had better had a watch on him or her, as in the darkness there is no depth perception and impossible to gauge the speed of the actual animal via the naked eye.
I have had number of circumstances where I have gotten a horse from the pole in the dark, at Keeneland, Saratoga, Oklahoma training center, Palm Meadows training center and others,u thinking the horse is flying, then you look at watch and its 1:03, or :50.2.
Of course, there is exceptions to the rule, but overall, works in the dark are not as fast as people think, and it is much tougher to assess movement, and marks. Trainers have to rely on the exercise rider ability to give a meaningful appraisal of the work.
So, when someone tells you the horse has been working in the dark, you have some knowledge of what it all means.