The physics of soft materials can be investigated using nuclear spin-lattice relaxation, which depends on the spectral densities of motion in the MHz range. For the first time, NMR relaxation has been used to study influences of the acyl length, polar head groups, a cosurfactant, and cholesterol on the viscoelastic properties of membrane lipids. The results imply the concept of elastic deformation is relevant on lengths approximately equal to the bilayer thickness and less, involving a broad spectrum of collective modes which contribute to the forces between lipid bilayers.