Many individuals tend to sleep-in later over the weekend, possibly in an attempt to catch up on accumulated sleep loss from the working week. Previous studies have found that delaying bedtime and waking-up time (WUT) results in a delayed circadian rhythm and a decline in subsequent mood and cognitive functioning. The present study investigated the effect of delaying only WUT for two weekend mornings on the timing of the dim light melatonin onset circadian rhythm (DLMO), as well as sleep, daytime sleepiness and fatigue. In a repeated measures design with 16 participants, the delayed weekend WUT condition (DS) was compared with a weekend in which they kept their habitual weekday WUT (HS). On average, participants in the DS condition delayed their weekend WUT by about 3 h. When compared to the HS condition, participants in the DS condition had a significant delay in salivary DLMO of 42 min between Friday and Sunday nights. They also indicated an 8-min increased sleep onset latency on Sunday night and significantly greater daytime fatigue and sleepiness on Monday and Tuesday of the following week. Sleeping-in late on the weekend appears to have a subsequent cost of delaying circadian rhythm, delaying sleep on Sunday night and increased daytime sleepiness and fatigue, the so-called ‘Monday morning blues’.