For many spells that require rolls, the number of dice is listed under the effect listing of the character sheet, making it really easy to roll the correct number of dice - especially for spells that scale up when you cast them at higher levels. 

However, for the Sleep spell, while the dice are explained in the spell description, the only thing listed under effect is "Unconscious."  Even when pressing the "cast" button for this spell, it only checks off a spell slot.  Even with the new dice feature, you have to manually add up the number of dice and roll it, or click the 5d8 and then the correct number of 2d8s from the spell description.

Am I missing something?  Otherwise, there should really be an easier way of doing this.

This is something I hope they are working on (the roller is currently in beta so things are still being added) but for now, all you have to do is in the lower left corner when you select to roll d8s, just left click the number of d8s you need to roll, say 5d8 so click 5 times, and the roller will roll 5d8 and add it for you