Internet-delivered treatment is effective for insomnia, but little is known about the beneficial effects of support. The aim of the current study was to investigate the additional effects of low-intensity support to an internet-delivered treatment for insomnia. Two hundred and sixty-two participants were randomized to an internet-delivered intervention for insomnia with (n = 129) or without support (n = 133). All participants received an internet-delivered cognitive behavioral treatment for insomnia. In addition, the participants in the support condition received weekly emails. Assessments were at baseline, post-treatment, and 6-month follow-up. Both groups effectively ameliorated insomnia complaints. Adding support led to significantly higher effects on most sleep measures (d = 0.3-0.5; p < 0.05), self-reported insomnia severity (d = 0.4; p < 0.001), anxiety, and depressive symptoms (d = 0.4; p < 0.01). At the 6-month follow-up, these effects remained significant for sleep efficiency, sleep onset latency, insomnia symptoms, and depressive symptoms (d = 0.3-0.5; p < 0.05). Providing support significantly enhances the benefits of internet-delivered treatment for insomnia on several variables. It appears that motivational feedback increases the effect of the intervention and encourages more participants to complete the intervention, which in turn improves its effectiveness.