This is in response to both Zulie and Pavane:
I definitely don't feel people should feel guilty of forming other streams of income, but it really should be done on your own time. That said, I am sure I have "cheated'" on this, myself. But, I generally give anyone I work for more than 100% - team player, innovated thoughts on how to do things better, step up to different forms of management, exceed goals, etc. Feeling stuck because of the need for the benefits is a decision each one of us has to make for ourselves. I have had healthcare premiums of over $700 a month until I got on Medicare.
If you can do the job you are doing in 20 hours vs the 40 hours you signed up for and accepted, then you have made a contract with the employer and are breaking that contract. Integrity says you should tell them that you can do it in 20 hours, and renegotiate a salary that fits the both of your needs. Depending on the employer, maybe if they are saving money in salary, and they now see you as a person with integrity, they may be willing to offer something else - like healthcare. In my case one year, and because they wanted to keep me on, an employer offered to pay half of the $700 premium, which I accepted. They did so for over a year until I got on Medicare. My bosses DID care about me and I cared about them, too. Yes, we both provided something needed and important for each other. That's what negotiations are about. If you can't come up with a negotiation that works for the both of you, then it is over.
It would be naive to think that they won't figure out what you are doing, on the side and you end up losing your job that day. Be prepared for that because it could be a reality; therefore, you must be willing to walk that day. You really aren't outsmarting them.
P.S. I have worked every weekend for nine years, unless I took a time off. But, I take off three days during the week and do my side hustle then. There are always options.