Quick answer: Your dog isn't barking at nothing — it's responding to sounds, smells, or stimuli beyond human perception. Dogs hear frequencies up to 65,000 Hz and smell up to 100,000 times better than humans.
Dogs hear up to 65,000 Hz (humans cap at 20,000 Hz) and have a sense of smell 10,000-100,000 times more sensitive. Barking at walls frequently indicates rodents, pests, or pipes. Barking at the sky responds to distant sirens, aircraft frequencies, or wildlife sounds. What's invisible to you is entirely real to them.
Many dogs are hard-wired to alert their household to unusual stimuli — it's a guarding instinct. If barking is brief, your dog looks at you, and then settles, it's functioning normally. Repeated, sustained barking without settling may indicate boredom or anxiety rather than a specific stimulus.
Dogs over 10 that have recently started barking at nothing — particularly at night — may be showing signs of canine cognitive dysfunction syndrome (dog dementia). This causes disorientation and vocalization, especially in quiet nighttime environments.
Dogs barking at nothing are responding to something real to them. Brief alert barking is normal. Sustained unexplained barking — especially at night in older dogs — warrants a vet conversation.