Cholesterol is a key molecule in controlling membrane fluidity, packing state, and various functions in eukaryotic cell membranes. It also regulates antibiotic drug resistance by augmenting the mechanical properties of its host membrane against structural damage. While, structurally, cholesterol exhibits universal condensing effects on fluid lipid membranes (saturated and unsaturated), its effects on membrane mechanics are presumed to be non-universal. Specifically, cholesterol causes strong stiffening in saturated lipid membranes, yet its mechanical effects on unsaturated lipid membranes remain controversial. Previous studies on di-monounsaturated lipids, such as DOPC, have shown that cholesterol has little effect on the bending rigidity of such membranes, despite clear evidence of a condensing effect. In contrast, neutron spin echo (NSE) spectroscopy shows that cholesterol yields a non-trivial stiffening of DOPC membranes over NSE timescales [1]. Here, we report similar observations using solid-state 2H-NMR spectroscopy. The model-free functional dependence of spin lattice relaxation rates (R1Z) on segmental order parameters (SCD), unambiguously indicates increased membrane bending rigidity with cholesterol [2]. These results are contextualized within the fluctuation-dissipation theorem which connects relaxations from thermal fluctuations in liquid-crystalline membranes to quasi-elastic deformation on mesoscopic length scales. Parallel studies on membranes with different lipid unsaturation (e.g. POPC vs. DOPC) indicate that cholesterol-induced stiffening is dependent on chain unsaturation and is present in all studied membranes. Our dynamical results - as well as structural SANS/SAXS and ESR data - are in excellent agreement with real-space fluctuation analysis from molecular dynamics simulations on cognate DOPC-cholesterol bilayers and with standard theories of membrane mechanics. Our findings have significant biological implications and encourage reassessment of mechanical measurements of cholesterol-containing membranes over length and time scales relevant to biological processes. [1] R. Ashkar et al. Biophys. J. (2019) 116, 328a. [2] T.R. Molugu et al. (2016) Chem. Phys. Lipids 1848, 246.