Hair growth is a biological process that happens at the root of the hair follicle, under the scalp. It's driven by cell division and protein synthesis. Whether your hair is in a ponytail or not doesn't directly affect the rate of this process.
Here's why the idea is a misconception:
* Hair growth is continuous (with cycles): Hair grows in cycles (anagen, catagen, telogen, exogen). The anagen phase (growth phase) can last for years. While hair growth might seem more noticeable over time, it doesn't happen suddenly just because it's been in a ponytail or because it's nighttime.
* Ponytails can cause breakage (potentially the opposite effect): If a ponytail is too tight, it can cause stress and breakage along the hair shaft. This can give the illusion of less growth over time, as the ends are snapping off.
In summary: Hair growth is a biological process, and ponytails themselves don't stimulate or inhibit it (though tight ponytails can lead to breakage). The idea that hair grows in a ponytail at night is a myth.