If four hours of sleep cannot be guaranteed, the brain will not be well rested; if the brain is not well rested, most people will feel mental fatigue the next day. So it can be inferred? ['as long as the brain is fully rested, mental fatigue can be eliminated.', "most people's spirit comes from lack of sleep.", 'if you only sleep for three hours, you will be mentally fatigued the next day.', 'if the brain is well rested, four hours of sleep must be guaranteed.']