Yes, it is true. Just one good joke in a day can actually relieve a high amount of stress, and make your hormones come to their normal levels despite all the anxiety and pressures of the professional life. Think of it this way, has not there been a day when you were told a joke by a friend or relative on the phone or via SMS, and you kept on thinking about the same throughout the day, and had a great, stress-free working experience. You might also have spread the cheer by telling the joke to your colleagues, thereby also brightening their day and bringing some laughter in their lives. Is not this proof enough of what a great joke can do to your entire day?