Time-of-day as a critical biological variable

Abstract

Time-of-day is a crucial, yet often overlooked, biological variable in biomedical research. We examined the top 25 most cited papers in several domains of behavioral neuroscience to determine whether time-of-day information was reported. The majority of studies report behavioral testing conducted during the day, which does not coincide with the optimal time to perform the testing from an functional perspective of the animals being tested. The majority of animal models used in biomedical research are nocturnal rodents; thus, testing during the light phase (i.e. animals' rest period) may alter the results and introduce variability across studies. Time-of-day is rarely considered in analyses or reported in publications; the majority of publications fail to include temporal details when describing their experimental methods, and those few that report testing during the dark rarely report whether measures are in place to protect from exposure to extraneous light. We propose that failing to account for time-of-day may compromise replication of findings across behavioral studies and reduce their value when extrapolating results to diurnal humans.

Keywords: Aggression; Attention; Circadian rhythms; Diel; Drug seeking behaviors; Food intake; Learning; Mating behavior; Memory.

Figure 1.

Mean latency (sec) to fall from a 2 cm pole during the day (rest phase) did not differ between wild-type (WT) and nNOS−/− mice (Nelson et al., 1995). However, latency to fall was increased for WT mice when tested during the night (active phase) suggesting a floor effect when tested during the day (Kriegsfeld et al., 1999).

Figure 2.. Time-of-day reporting in eight behavioral neuroscience domains.

(A) Frequencies of behavioral testing timing reports from the top 25 cited articles in eight domains of behavioral neuroscience. Non-review articles from 2000–2020 were identified using Web of Science search queries and were cataloged according to reporting within the manuscript. (B) A pooled summary of reporting percentages across the eight examined domains reveals that over half of the 200 examined articles either failed to report timing of behavioral testing or had ambiguous reporting.