This is up to your personal preference. Please do not place the Z1™ or the Z2™ on any surface that could block the intake of air (fluffy carpet, under the covers, etc.) and do not place the Z1™ or the Z2™ in any position that could cause the hose to kink.
Your nightstand, over the headboard, or on the floor beside your bed (if not carpeted) are all places you can let your Z1™ or Z2™ sit while you are using it.