Your newborn's brain is doing extraordinary work in the first four weeks, but almost none of it is visible. They're processing sensory information for the first time — light, sound, touch, temperature, gravity. Their nervous system is immature, which is why they startle easily (the Moro reflex), have jerky movements, and need help regulating their body temperature and state.
Sleep is where most of the brain's development work happens. About 50% of a newborn's sleep is active (REM) sleep, compared to about 20-25% for adults. During REM sleep, the brain is building neural connections at an incredible rate. This is why newborns sleep so much — their brain literally needs it for construction.
The startle reflex can disrupt sleep significantly. A tight swaddle (arms in, hips loose) mimics the snug feeling of the womb and dampens the reflex. Most newborns sleep noticeably better when swaddled. Just make sure to stop swaddling when your baby shows the first signs of rolling, which can happen as early as 2 months.