This study considers the role of pre-sleep cognitive arousal, worry, and inhibition in sleep onset difficulties. The Pennebaker writing task, which promotes emotional processing by asking people to write about their thoughts, worries, and emotions, has proven effective in several areas of health. Here, the paradigm's ability to reduce pre-sleep cognitive arousal (PSCA) and sleep onset latency (SOL) in people with insomnia was tested. Twenty-eight people with insomnia were randomized to three nights of Pennebaker writing or a control condition, following a one-night baseline. The outcomes of change over baseline at Day 4 in pre-sleep cognitive arousal and SOL were compared. Writing significantly reduced pre-sleep cognitive arousal on one out of two measures, but did not significantly reduce SOL.