<p>Vascular dynamics in the eyes and brain under microgravity have gained attention due to the Spaceflight-Associated Neuro-Ocular Syndrome (SANS). We explored how changes in gravitational vectors, circadian rhythm, and sleep regulate cerebral blood flow (CBF) and eye perfusion. Hypothesizing that the eye lacks the capacity to autoregulate, we expected increases in blood flow or volume to drive choroidal engorgement in SANS, and circadian and sleep-related changes to further influence these dynamics. Lower-body negative pressure (LBNP) was investigated as a countermeasure for posterior ciliary artery velocity (PCAv) changes. 16 participants were examined in seated, supine, lateral, and prone positions to assess blood flow in the internal carotid and vertebral artery, and PCAv and middle cerebral artery velocity (MCAv) via ultrasonography. CBF and PCAv were stable in-between horizontal positions, and both lowest in a seated position: CBF (supine, 1036 ± 231 vs. seated, 889 ± 177&#xa0;ml∙min^− 1; <i>P</i> = 0.0019), PCAv (supine, 7.24 ± 1.8 vs. seated, 5.46 ± 1.3&#xa0;cm∙sec^− 1; <i>P</i> = 0.0012). CBF conductance was lower in the seated position (<i>P</i> = 0.0303), but after correcting for intraocular pressure and hydrostatic columns, PCA conductance remained stable (<i>P</i> = 0.0876). Oxygen delivery was higher in the supine position (supine, 212 ± 54 vs. seated, 182 ± 41&#xa0;ml∙min^− 1; <i>P</i> = 0.0022), but unchanged between horizontal positions (both <i>P</i> &gt; 0.29). No diurnal changes were observed (all <i>P</i> &gt; 0.15). Sleep decreased MCAv (<i>P</i> &lt; 0.0001), heart rate (<i>P</i> = 0.0019), and mean arterial pressure (<i>P</i> = 0.0019). LBNP unexpectedly increased PCAv (<i>P</i> = 0.0358) and ocular perfusion pressure (<i>P</i> = 0.0156). In conclusion, CBF and PCA velocity change similarly with postural changes on Earth, and low-level LBNP was ineffective in lowering PCAv.</p>