This paper presents a dynamically adaptive proximity controller (APC) to balance energy consumption and user comfort of computer screens in office environments. Our APC system detects desk activities, such as working with the computer screen (screen on) and being away (screen off) and controls screens accordingly. Ultrasound range (USR) sensors were used to measure user proximity. To compensate for USR measurement errors, APC timing parameters were dynamically adapted and previous screen switch-off operations corrected using implicit user feedback. The feedback was obtained from proximity variance increases due to user movement following erroneous control operations. System performance and user comfort were evaluated in a real-life intervention study with 12 participants during 19 days. Detection accuracy was up to 98 %. Energy savings of up to 21 % were obtained by comparing intervention and baseline measurements. User responses showed that the APC system could yield energy savings, while maintaining user comfort when assessed using pre-and post-intervention questionnaires. The implicit feedback control is suitable to reduce system commissioning effort.