Are you having anxiety during this or simply having numb legs? If it's the former some of the physical symptoms of anxiety can be mild and temporary numbness in the limbs, most commonly hands and feet. Otherwise it may be a circulation issue that may have ab underlying cause like POTS, you may be able to use compression socks and/or weighted blankets to help with the numbness, realistically with either of these situations. If any of this is alarming information for whatever reason such as no prior condition to explain it, hereditary heart conditions running in your family, or the symptom is new you may want to speak to a doctor about it.

Another thing it could be is Peripheral Neuropathy, it can be caused by something as simple as a Vitamin B12 deficiency. I think this is less likely, but that is what causes similar issues for me.