Cosinor analysis can detect changes in phase, amplitude, or means of circadian rhythm of body temperature (CRT). Two experiments (E1: 22 Angus heifers, 292 ± 9 kg BW; E2: 16 Angus and 8 Hereford × Angus heifers, 335 ± 11 kg BW) evaluated changes to CRT due to a minor, chronic stressor (endophyte) and a major, acute stressor (LPS). Diets, starting 10 d before LPS challenge and fed at 1.8 × NE_m in individual stalls (3.0 × 3.7 m), contained 20% of either endophyte-free (E-) or endophyte-infected (E+) fescue seed, 30% cottonseed hulls, 36% cracked corn, and 14% supplement and were balanced to meet protein and mineral requirements. E2 included MGA (0.5 mg/hd/d) and evaluation of weaning exit velocity (wEV).Vaginal (E1 and E2) and rectal (E2 only) temperature probes recorded body temperature every 5 min. Cosinor analysis was used to analyze data in each of 4 phases: pre-treatment diet (P1), treatment diet, pre-LPS (P2), post-LPS temperature spike 1 (P3), post-LPS temperature spike 2 (P4). Disruption to CRT occurred during P2 of both experiments, as evidenced by 39% of curves failing to fit a cosine model (i.e., less than 40% R²) compared with only 5% of curves failing to meet inclusion criteria for all other phases, suggesting that abrupt dietary shifts can affect temperature regulation. Amplitude differences across days (P = 0.05) occurred in P1 of E1, when mean temperatures did not differ (P > 0.10). Amplitude (P = 0.80) and mean temperature (P = 0.66) between treatment groups in P3 of E1 were similar, but shifts in timing of the peak (P = 0.09) occurred. Endophyte affected vaginal temperature amplitudes in P2 (P = 0.07) and P4 (P = 0.08) of E2 without influencing mean temperatures. Shifts in timing of peak vaginal temperatures during P1 (P = 0.01) of E2 across days may indicate adaptation of CRT to shifts in environment. Treatment × wEV interactions were significant for mean rectal, but not vaginal, temperatures during P1 (P = 0.10), P2 (P = 0.10), and P3 (P = 0.04) of E2, indicating different temperature responses between sites. These data demonstrate that cosinor analysis can be used to detect shifts in CRT responses that would not be detected by evaluation of mean temperature responses alone.