ABSTRACT.Optical sensors are a promising approach for assessing nitrogen (N) status of vegetable crops. However, their potential may be undermined if time of day influences measurements. This study evaluated the effects of time of day and N addition on measurements, made with two chlorophyll meters, SPAD-502 and MC-100, and two active canopy reflectance sensors, GreenSeeker handheld and Crop Circle ACS-470. Three treatments (N1, deficient, N2, conventional, and N3, excessive N application) of N concentration in the nutrient solution were applied by fertigation throughout a sweet pepper crop grown in soil in a greenhouse. Time of day of 12:00 and 15:00 h had an effect on measurements made with the SPAD-502, but only in the N1 treatment, suggesting that the effects of time of day were related to crop N status. This effect was slight, being 1.7 ± 0.02 SPAD units lower at 12:00 and 15:00 h compared to at 9:00 h (relative decrease of 3.6%). For the MC-100, a slight increase in Chlorophyll Content Index (CCI) values of 3.3 ± 0.1 units (relative increase of 6.3%) was observed at 15:00 and 18:00 h, relative to CCI values at 9:00 h, regardless of N treatment. The time of day effect on chlorophyll meters appears to be negligible in relation to the wide range of values measured in greenhouse-grown sweet pepper. Normalized Difference Vegetation Index, measured both with the GreenSeeker and Crop Circle, and Green Normalized Difference Vegetation Index, measured with the Crop Circle, were not affected by time of day in any of the N treatments, showing that these sensors and indices can be used with confidence at any time of the day. © 2019, The Author(s).