Monotony has been studied in areas such as assembly lines, power plant control rooms and driving vehicles yet these findings are not easily transferable to air traffic control. A simulator experiment with a generic sector design, and a field study in an ATC operations room was therefore conducted to examine the effects of traffic repetitiveness and traffic complexity on monotony as an operator state, applying subjective measures of attention or alertness levels (including fatigue, workload, attentiveness, concentration), together with physiological and performance indicators. The results suggest that an operator state of monotony is a consequence of repetitive conditions, reinforced by low dynamic density periods and dependent on individual factors. Combining some of these indicators, we propose a composite indicator for an operator state of monotony. We describe potential impacts of monotony and recommend both concept development strategies to minimize monotony in future ATC systems and corrective actions to alleviate monotony in existing operational environments.