Racing thoughts and worry activate the sympathetic nervous system (fight-or-flight), releasing cortisol and adrenaline, making relaxation and sleep onset difficult. The prefrontal cortex remains active when it should be quieting down.

Irregular sleep schedules, screen use before bed (blue light), stimulating activities, and uncomfortable sleep environment interfere with natural sleep onset and circadian rhythms.

Misaligned internal body clock (suprachiasmatic nucleus) with desired sleep schedule makes falling asleep difficult. Common in shift workers and those with delayed sleep phase.

Caffeine and nicotine are stimulants that block adenosine (sleep-promoting neurotransmitter). Alcohol initially sedates but disrupts sleep architecture later in the night.

Uncomfortable sensations in legs (and sometimes arms) create an urge to move, preventing relaxation and comfortable sleep onset.

Noise, light, temperature, or uncomfortable bedding can prevent the brain from associating the environment with sleep.

Chronic pain, GERD, asthma, thyroid disorders, and other conditions can make comfortable sleep difficult.

Depression, PTSD, OCD, and other psychiatric conditions can cause hyperarousal that interferes with sleep onset.