Because they sleep more deeply and have more of the bursts of brain activity known as sleep spindles. Everyone’s sleep differs, even though we all pass through the same four stages of non-REM sleep and several periods of REM (rapid eye movement) sleep every night. In the waking brain, a large area called the thalamus acts as a way-station for sounds, sights and other stimuli coming in, but during sleep it helps suppress them.
Patterns called sleep spindles, that can be seen using an electro-encephalogram, reveal the start of non-REM sleep. Recent research has found that really deep sleepers – those who will ‘sleep through anything’ – have more sleep spindles than the rest of us. Whether you can increase your sleep spindles is still unknown.