Model monopiles with outer diameter D = 50 mm are loaded laterally at 100×g in a large-beam geotechnical centrifuge. To study the effects of the embedded length L on the lateral response of monopiles, four lengths have been tested: L = 150, 250, 350 and 450 mm, corresponding to slenderness ratios of L/D = 3, 5, 7 and 9, respectively. The normal strains on both the tensile and compressive sides are measured using fibre Bragg gratings. Several findings have been observed. The monopiles generate asymmetric tensile and compressive strains during bending. This tension-compression asymmetry is more pronounced at the pile toe and for shorter piles. The piles transition from flexure to rotation as the embedding depth is decreased from 9D to 3D, where the uniqueness of the ground-level rotation and deflection (Og-yg) relationship disappears. Finally, the P-y curves flattens with increasing embedded depth.