No need to buy the light-blocking curtains. Jean Jacques d’ Ortous de Mairan’s 1729 experiment already demonstrated that the 24-hour circadian rhythm is not dictated by the dark/light cycle and sunlight. It is caused within the biology of organisms. If we take ourselves out of the outer world, we will still continue that rhythmic behavior. And, the French geophysicist showed this by  conducting an experiment on the leaf movements of a heliotrope plant over 24-hour periods. As the day time arrives, the plant unfolds its leave; yet, when darkness comes, the leaves collapses. Initially, everyone thought this behavior was governed by external forces (i.e. sunlight). However, Jean discovered that the leaves still exhibit this same behavior when he placed the flower away from the sun in a dark room. His experiment proved that circadian rhythm still persists in absence of external cues like light.  

(de Mairan, 1729)