Once henna paste dries, it tends to crack and crumble off, however henna must remain on the skin for 4 - 8 hours for the darkest and longest lasting stain. So in order to help prevent the paste from crumbling off, try sealing the henna to the skin with a lemon/sugar mixture.
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If you are going to sleep with the paste on, sealing it and then wrapping it in a tensor bandage, sock or something else breathable (that you don't mind getting stained), is a good way to prevent crumbs from falling off in the bed.