Is sleeping at noon useful or harmful?

Cultural: In spain and italy siesta is widely practiced. Therefore it cannot be bad.

Useful: Studies have shown that productivity increase after afternoon nap.

Neither and both: Adequate daily sleep and rest is a key bodily need. Some evidence suggests that night-shift work, and need for day sleep, is associated with increased health risks and symptoms. Log your sleep, wake, and exercise patterns - do you see a difference in your focus, energy and productivity. Review the logs with your doctor to see how your sleep cycles may be impacting your health.

Sleep: Most studies show sleep requirement is between 6 to 8 hours, more or less causes morbidity or mortality as long as you average this amount and feel refreshed you should be fine.