The answer is NO,. You should not keep your rice porridge or congee overnight. I know, logically there does not seems to be anything wrong with storing cooked porridge overnight.
The main reason why porridge or congee should not be kept overnight is because it would cause wind buildup in your body from the traditional Chinese healing perspective. In fact, I have been told that if you buy a packet of porridge to eat for lunch but could not finish it, you should not keep it for dinner.
Once, I remember a colleague of mine was unwell but still came to work. She bought a packet of porridge for lunch. As she was too busy with work, the packet of porridge laid on her desk until she had finally had time to eat it in the evening. The porridge was already cold. After eating, she complained that her stomach became very uncomfortable.
Keeping overnight or more than that is out of the question.
The very nature of rice porridge or congee is that it very good to soothe a queasy stomach and to be taken when we feel unwell, flu or simply have no appetite. But as the hot porridge starts to cool down, its disposition would change.
I have personally suffered from a wind in my stomach after heating and consuming overnight porridge. Basically the porridge was boiled with quite good ingredients and I felt it was wasteful to throw away. I put in the fridge when I could not finish it. The next day I took it out, heat it up and take- and I suffered the opposite effect. When the day before I felt the porridge soothed my stomach, the next day I felt my stomach going queasy after taking it.