Overnight meals are most likely to produce nitrite, which causes poisoning. Although the overnight meal may only be stored for one night, it has been preserved for a long time. Microorganisms in the air sneak into meals and produce reductase, which can turn nitrate into nitrite.
The reason why this nitrite can make people toxic or even die when taken in small amounts is because it can convert normal hemoglobin in blood into methemoglobin which can't transport oxygen, and it is naturally very harmful to human body.
In addition, if you eat overnight meals for a long time, it may increase the risk of gastric cancer. Nitrite bred in overnight meals can react with protein decomposition products contained in food in acidic environment, and can generate carcinogens. If everyone eats overnight meals, it is undeniable that the possibility of getting gastric cancer will be much higher than others.
Overnight meals can also breed many bacteria invisible to the naked eye, such as the most common putrefying bacteria. This kind of bacteria likes nutritious meals best, which can release acidic gas that greatly increases appetite, making people fall in love with this taste unconsciously. People who eat overnight meals contaminated by bacteria are prone to gastrointestinal diseases and discomfort.
To sum up, everyone should try to eat freshly baked meals. If you save leftovers, you can put them in a clean fresh-keeping box in time after cooking, and put them in the refrigerator after the food is not so hot.