High salt intake and hypertension are thought to be linked. The steady-state concept of sodium balance implies that Na+ accumulation leads to water retention while salt intake correlates with rapid urinary excretion. Rakova et al. studied cosmonauts in Mars space flight simulations over 105 and 205 days and found that Na+ excretion and retention exhibit hormone-dependent rhythmic patterns unrelated to blood pressure, body water, or salt intake. The cover image highlights the importance of the kidneys in sodium balance, which is under clock control. Mars, sodium ions, and atomic models are represented in the background.