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<p>During the Late Pleistocene, Dansgaard–Oeschger (DO) cycles triggered warming events that were as abrupt as the present-day human-induced warming. However, in the absence of a periodic forcing operating on millennial timescales, the main energy sources of DO cycles remain debated. Here, we identify the energy sources of DO cycles by applying a bispectral analysis to the North Greenland Ice Core Project (NGRIP) oxygen isotope (<span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_ice</span>) record; a 123 kyr long proxy record of air temperatures (<span class="inline-formula"><i>T</i>_air</span>) over Greenland. For both modes of DO cyclicity – slow and fast – we detect disparate energy sources. Slow DO cycles, marked by multi-millennial periodicities in the 12.5 to 2.5 kyr bandwidth, receive energy from astronomical periodicities. Fast DO cycles, characterized by millennial periodicities in the 1.5 <span class="inline-formula">±</span> 0.5 kyr range, receive energy from centennial periodicities. We propose cryospheric and oceanic mechanisms that facilitate the transfer of energy from known sources to slow and fast DO cycles, respectively. Our findings stress the importance of understanding energy-transfer mechanisms across a broad range of timescales to explain the origins of climate cycles without primary periodic energy sources.</p>