Rest cycle

While being on Earth, the Sontarans needed a rest cycle of 7.5 minutes every 27 hours.

The Sontarans had to wear the protective armoured suits, because they couldn't cope with Earth's atmosphere over an extended period. The suits filtered out harmful Earth gases and substituted them with chemicals and nutrients from their home planet. But the suits only had a limited capacity and had to be refilled. Rest cycle was a period during which the Sontarans toped up their suits by the probic vent in the back of their necks, resting and refuelling.

In an Alternate timeline, that was created by the Sontarans during their Flux assault, The Thirteenth Doctor used this feature in her Project Crimean Eviction. (TV: War of the Sontarans ^[+]Loading...["War of the Sontarans (TV story)"])