Yoga Nidra, also known as "yoga sleep," is a practice that involves a body scan and deep relaxation. It's not so much a focused meditation, but more about focusing on the sensation of the body and trying to reduce the activity of the prefrontal cortex. It's been shown to replenish levels of certain neuromodulators like dopamine and reduce cortisol, a stress hormone.
There's a similar practice called NSDR, or non-sleep deep rest, which is like Yoga Nidra but removes some of the mystical language and intentions, focusing more on the physiology and body scans. Both practices can be very effective at improving focus, especially under conditions of sleep deprivation. They can also help with sleep issues, as shown in a study where Yoga Nidra improved sleep in patients with chronic insomnia.
You can find scripts for both Yoga Nidra and NSDR on YouTube and other platforms at zero cost.