There was a study about this in 2014. It’s entitled “Analysis of pain level in cases treated with aligners: comparison with fixed edgewise appliance therapy” (Fujiyama et al., 2014). In this study, researchers asked 145 volunteers who were having their crooked teeth straightened to identify the pain quotient on a day-to-day basis using a scale. They were asked to keep a diary to record their experience. They were divided to 1 groups: conventional braces and aligners. The results showed a significant difference in pain intensity between those who were using aligners and those who were using braces. They also found that the duration of discomfort and pain lasted longer with patients who wore braces.
Another study related to this is “A comparison of treatment impacts between aligners and fixed appliance therapy during the first week of treatment” (Miller et al., 2007). This studied the impact it had on the patient’s quality of life. The researchers took 60 patients and asked them to keep a diary of their treatment. 33 wore aligners in the group. The volunteers who wore aligners found that the impact of treatment on their quality of life was minimal. They also found that patients who wore aligners experienced significantly less pain-related issues than those who wore braces.