Feeling nauseous at night can be caused by several factors, most commonly acid reflux or gastroesophageal reflux disease (GERD). This happens when stomach acid flows back into the esophagus, especially when lying down after a big or late meal. Other common causes include anxiety or stress, which tend to worsen at night when distractions are fewer, leading to gastrointestinal discomfort. Sleep disorders, stomach ulcers, pregnancy, medication side effects, and conditions like gastroparesis can also contribute to nighttime nausea. Managing meal timing, reducing stress, elevating the head while sleeping, and addressing any underlying health issues can help reduce this symptom.