dormir sur ses deux oreilles

You can use it literally to describe a great night. However, it is most common as a metaphor for peace of mind. You will often hear it in the future tense or imperative. A friend might say, Tu peux dormir sur tes deux oreilles. This means "You have nothing to worry about." It functions like a verbal weighted blanket. Just conjugate dormir and change the possessive adjective ses to match the subject.

Use it when you want to reassure someone. If your friend is nervous about a house-sitter, tell them they can dormir sur leurs deux oreilles. Use it at work when a project is finally finished and perfect. It is great for those moments of relief after a long period of stress. It fits perfectly in a text message to a partner. It also works well when discussing a reliable person you trust completely.

Avoid using this in high-stakes, tragic situations. It is a bit too cozy and lighthearted for a funeral or a major crisis. Do not use it if you are actually talking about someone who is lazy. It implies peace, not lack of ambition. Also, do not use it if someone is literally having ear surgery. That might be a bit too literal and awkward for everyone involved!

This phrase dates back to the 17th century. The logic is wonderfully absurd. Physically, you can only sleep on one ear at a time. By claiming to sleep on both, you are achieving the impossible. It suggests a level of comfort that defies the laws of physics. It reflects the French value of tranquillité. In French culture, being able to sleep well is a sign of a life well-lived and a clear conscience.

You might hear dormir comme un loir (sleep like a dormouse). Another common one is dormir comme une marmotte (sleep like a marmot). If you want to be more intense, try dormir à poings fermés. This means sleeping with clenched fists, or very deeply. But for pure emotional reassurance, dormir sur ses deux oreilles remains the gold standard. It is the most comforting of the bunch.