Body control! Remna have nerve clusters distributed all around their body that allow them to consciously control what would normally be autonomic functions. You can use these to diagnose and treat diseases and injuries, but one of the uses of body control is to induce various types of dormancy. Old-Paril's memories will have the time he tried aestivation and hibernation for the first times, as a child. No one does that anymore, even back then, because they have good shelter and enough food for everyone, but it's considered Culturally Important that children still be taught body control dormancy techniques. Aestivation and hibernation protect from hot and cold temperatures, and also greatly reduce the need for food and water while dormant. It's not without risk, though, the body is at risk of infection during these times, so it's important that the place where people go dormant be kept very clean. Also, they're going to be really hungry and thirsty and tired for hours to days afterward as they recover.
Sleep is the simplest type of dormancy, and is the only mandatory one. Remna must sleep at least for some portion of the day: usually six, but can go as low as four if you've done little thinking or moving in the day, or up to twelve, for drones doing grueling work like pulling carriages quickly by running. People can also sleep longer than that to recover from disease or injury, in which case they may remain in sleep for most of the day, waking only to eat, drink, and relieve themselves. Sleep requires no complex technique – if you get tired enough you'll actually go to sleep against your will – but you might need to do something extra if you have to sleep under adverse conditions, such as changing your sleep schedule, or the room being bright or loud.
Unlike other body control techniques, this one doesn't require poking and prodding, and instead only requires mental motions. Old-Paril will have many memories of his favorite one to induce sleep.