Seasonal Differences in Biological Rhythms of Brazilian Native Indians

*Corresponding author e-mail; danicrono@gmail.com Introduction/Objective: Compare summer and winter data of wrist temperature and activity/rest pattern of adult indians who live in a settlement without electricity. Methods: Participants were 22 Guarani adult indians aged 18 to 80. The settlement “Boa Vista” is near the Atlantic coast in southeast Brazil (23°21’S; 44°51’W). Photophases during data collections were approximately 13h in summer and 11h in winter. Wrist temperature (WT) were collected every 30min along three consecutive days with thermistors (Thermochron., iButton type DS 1291H). Rest/activity (RA) was registered with actimeters (MicroMini Motionlogger. - Ambulatory Monitoring, Inc.) for 7 consecutive days, in bins of 1minute. The data were analyzed with the COSINOR method and the parameters acrophase, amplitude, MESOR were compared in two moments. The Pearson moment correlation test was applied to analyze the changes in COSINOR parameters according to age. The onset, offset and duration of rest between seasons were compared with one-way ANOVA. Results: WT: All subjects showed a significant circadian rhythmicity (COSINOR; p<0.0001). The amplitude values were higher and MESOR values were lower in winter than in summer. Although submitted to distinct photophases, acrophase values did not differ according to season. RA: the indians showed a phase advance in winter compared to summer values (onset winter; 21h23 and onset summer; 22h42; F=18.69, p<0.00004) (offset winter; 06hl2 and offset summer; 07hl8; F=23.98, p<0.00001) without difference for rest duration. The phase relation between WT and RA varies according to the season; □ Ψwinter = 11 h 13 and □Ψsummer = 12h46. Conclusion: The adult indians showed WT rhythms with seasonal difference, specifically in the amplitude and MESOR. The RA was linked to specific characteristics of Guarani routine. In winter the indians’ activities began near the sunrise and during summer they used to phase-delay their rhythms probably due to social demands involving nighttime activities during summer. Publication History Article published online: 16 June 2026 © 2009. Brazilian Sleep Academy. This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/) Thieme Revinter Publicações Ltda. Rua Rego Freitas, 175, loja 1, República, São Paulo, SP, CEP 01220-010, Brazil