Aching legs at night have many causes. Common, benign ones include muscle fatigue from daytime activity, dehydration, and night cramps, which are sudden painful tightenings usually in the calf. Restless legs syndrome causes an uncomfortable urge to move the legs, often worse in the evening and relieved by movement.
Other causes include reduced circulation, nerve-related pain (such as from sciatica or peripheral nerve problems), and, in some cases, joint or vein conditions. Pain in one leg specifically, with swelling, warmth, and redness, can indicate a blood clot (deep vein thrombosis), which is a medical emergency.
Seek urgent care if one leg becomes painful, swollen, warm, or red, if you have sudden severe calf pain, or if leg pain comes with chest pain or breathlessness. For ongoing night leg aches without these features, see a doctor to look for treatable causes such as cramps, circulation problems, or nerve issues, and to review any medicines that might contribute. Staying hydrated and gentle stretching may help cramps.