12 hours and 1.65 hours Hours
||Human Homo sapiens
||Harrington MG et al., Cerebrospinal fluid sodium rhythms. Cerebrospinal Fluid Res. 2010 Jan 20 7: 3. p.1 3rd paragraph and p.4 right column 2nd paragraph and p.5 left column top paragraphPubMed ID20205754
||P.2 right column bottom paragraph: "Sodium and potassium cations were calibrated (and continuously verified) with NIST-certified standard mixtures
(SPEX CertiPrep, Metuchen, NJ, USA)."
||P.1 3rd paragraph: "The distribution of sodium varied much more than potassium, and there were statistically significant rhythms at 12 and 1.65 h periods." P.4 right column 2nd paragraph: "Data from 144 samples per subject over 24 h allows rhythms at 20 min or greater to be detected. The principal rhythm with a 12-h period is visible in Figures 4 & 5. [Investigators] have less confidence in the 24-h period, since this involved the entire time of [their] sampling. This would require further testing in a longer duration study. [They] did not confidently identify the other significant period at 1.65 h (100 min) in the raw data ([they] were not looking for it), but it is significant in all the spectral analyses (Figure 6 and Additional Files 1 and 2). The primary source of sodium is dietary, but since the two meal times of these participants did not coincide with the principal 12- or 1.6-h cycles, their diet is not likely to be responsible for these rhythms."