Peak olfactory acuity is 9pm... Maybe to help finding sexual mates.

The Influence of Circadian Timing on Olfactory Sensitivity. Rachel S Herz, Eliza Van Reen, David Barker, Cassie J Hilditch, Ashten Bartz, Mary A Carskadon. Chemical Senses, bjx067, https://doi.org/10.1093/chemse/bjx067

Abstract: Olfactory sensitivity has traditionally been viewed as a trait that varies according to individual differences but is not expected to change with one’s momentary state. Recent research has begun to challenge this position and time of day has been shown to alter detection levels. Links between obesity and the timing of food intake further raise the issue of whether odor detection may vary as a function of circadian processes. To investigate this question, thirty-seven (21 male) adolescents (M age =13.7 years) took part in a 28-hr forced-desynchrony (FD) protocol with 17.5 hours awake and 10.5 hours of sleep, for seven FD cycles. Odor threshold was measured using Sniffin’ Sticks six times for each FD cycle (total threshold tests = 42). Circadian phase was determined by intrinsic period derived from dim light melatonin onsets. Odor threshold showed a significant effect of circadian phase, with lowest threshold occurring on average slightly after the onset of melatonin production, or about 1.5 ○ (approximately 21:08 hours). Considerable individual variability was observed, however, peak olfactory acuity never occurred between 80.5 ○- 197.5 ○ (~02:22-10:10 hours). These data are the first to show that odor threshold is differentially and consistently influenced by circadian timing, and is not a stable trait. Potential biological relevance for connections between circadian phase and olfactory sensitivity are discussed.

Keywords: adolescents, food intake, forced desynchrony, individual differences, odor threshold, trait-state