In his recent (2017) book Justin Brierley asks:

Were the disciples just hallucinating that they saw the resurrected Jesus? Hallucinations do sometimes occur when people lose loved ones. The people most likely to experience a grief hallucination are senior adults grieving the loss of a spouse. Approximately 50 percent do, often believing they hear or sense the person with them. However, only 7 percent of all senior adults grieving the loss of a loved one experience a visual hallucination of that person. It's also worth noting that people don't experience the same hallucinations—most psychologists agree that mass hallucinations don't occur.

In contrast, 100 per cent of the disciples experienced what they believed were visual appearances of Jesus. That's a far greater percentage than can be supported by hallucination research completed during the past century. Those who propose a "hallucination theory" also generally assume the disciples must have had a powerful psychological incentive to see Jesus come back to life. However, the disciples had no pre-existing expectation of a rising Messiah-figure. Jesus' resurrection was unexpected and out of keeping with their Jewish theological expectations. The Gospels repeatedly mention the skepticism of the disciples.