Hi and welcome to the Chiari forum.
I am not familiar with a sunbed, do you mean for tanning? Any exposure to sun or imitation sun light should be discussed with your Dr as they would know how long exposure is safe for you with your skin type and if you have any other issues that could prove to be a reason to avoid the sunbed.
Chiari does come with other related conditions, so they should be ruled out to really know what is best for you. For example if you have POTS a sunbed may not be a good idea...it is one of the related conditions to Chiari....not everyone with Chiari develops it, but many do.
So, you really should consult your Dr regarding this question.