How does sleep deprivation affect emotional regulation?

Substantially, and faster than most people expect. Studies restricting sleep show increased reactivity in threat-processing regions and weaker regulatory control from the prefrontal cortex, which translates practically into a shorter fuse, more negative interpretation of neutral events, and less capacity to put things in perspective. Even a single poor night measurably increases next-day anxiety. This matters because emotional difficulty and poor sleep form a loop, and sleep is often the more tractable end. If someone is struggling and also sleeping four hours, addressing sleep first is rarely wasted effort.