<p>Many genes have nycthemeral rhythms of expression, i.e. 24-hours periodic variation, at either mRNA or protein level or both, often in a tissue-specific manner. Here we investigate the evolutionary trade-offs which can explain rhythmic expression in different genes and tissues. We test two main advantages for rhythmicity: cost saving on protein production and the control of expression noise. We find that cost saving explains rhythmicity at the protein level, while control of noise explains rhythmicity at the mRNA level. Trends for costs are consistent in bacteria, plants and animals, and are also supported in tissue-specific patterns in mouse. Noise control had strongest support in mouse, with limited power in other species. Genes under stronger purifying selection are rhythmically expressed at the mRNA level, probably because they are noisesensitive genes. Moreover, we suggest that mRNA rhythmicity allows to switch between optimal precision and higher stochasticity. This higher stochasticity allows to maintain oscillations and to exhibit diverse molecular phenotypes, i.e. "blind anticipation" of cells. The ability to alternate between these two states, enabled by rhythmicity at the mRNA level, might be adaptive in fluctuating environments. The adaptive role of rhythmic expression is also supported by rhythmic genes being highly expressed yet tissue-specific genes. This provides a good evolutionary explanation for the observation that nycthemeral rhythms are often tissue-specific.</p>