How effective are non-monetary incentives in relieving stress
That depends upon the nature and causes of the stress.
If an employee is stressed because of lack of money, then monetary incentives would help relieve that stress.
But most stress is caused by other factors. If an employee is stressed because of overwork, poor peer relationships, an overbearing boss, family problems, or a generally unfriendly work environment, then non-monetary incentives would certainly help relieve that stress.