Later in the experiment, the crew spent more time in bed or engaged in personal activity. The crew's overall activity levels plummeted in the first three months, and continued to fall for the next year. On their return journey, they spent 700 hours more in bed than on the outward journey. Four of the members suffered from sleep and psychological issues. One crew member slept so badly he suffered chronic sleep deprivation and single-handedly accounted for the majority of mistakes made on a computer test used to measure concentration and alertness.