Duning at Noon
Riding in sand is a lot different than riding on dirt. There are many reasons to take extra precautions or alter your riding technique when riding in sand, including traction, the softness of the sand, and something you might not have considered; shadows.
That's right, shadows play a major role in your eye's ability to define much of the terrain you ride on in the dunes. There are fewer color differences in sand compared to dirt, and the dunes are highly dynamic; they can change every day, especially in heavy winds.
So a one foot high abrupt ridge that wasn't there the day before, may have snuck up while you slept and cause you to have an over the bars episode if you don't see it. Take extra care when riding at noon, even in areas you've ridden the previous day.