We present the design and prototype of the Idle Stripes shirt, which is an aesthetic, clothing-integrated display, reflecting the wearer's physical activity in an ambient manner. The design is targeted to be smart business-wear for an office worker, which creates awareness of immobility periods during typical sitting-intensive office work. Long periods of such sitting are known to be health risks. The Idle Stripes shirt promotes healthy working, encouraging the wearer to break up their office desk work with walking breaks. The design prototype is constructed of a fabric with integrated optical fibers, which are illuminated based on the sitting time detected by an app running on the wearer's mobile phone.