As technology scales down at an exponential rate, leakage power is fast becoming the dominant component of the total power budget. A large share of the total leakage power is dissipated in the cache hierarchy. To reduce cache leakage, individual cache lines can be kept in drowsy mode, a low voltage, low leakage state. Every cache access may then result in dynamic power consumption and performance penalties. A trade-off between the amount of leakage power saved on one hand, and the impact on dynamic power and performance on the other hand must be reached. To affect this trade-off, we introduce "slumberous caches" in which the power level of cache lines is controlled with the cache replacement policy. In a slumberous cache, cache lines are maintained at different power save modes which we call "tranquility levels", which depend on their order of replacement priorities. We evaluate the effectiveness of this idea in the context of PLRU, LRU and MRR (Modified Random rep...