Is there a circadian component to mood variations in humans? I'm looking at this question about an Experience Sampling Method (ESM) app:
Longitudinal mobile mood tracking app with random reminders

The studies linked in the question deal with sampling of user experience throughout the day. 

I'm wondering if studies like ESM have found that there's a circadian component to human activity or moods fluctuation throughout the day?
Overview: It seems like quite a bit of research has been conducted with experience sampling methodologies looking at daily variations in mood and activity. It is an interesting question as to how best tease out the effect on mood of time of day per se and activities associated with time of day on variations. It also seems like there are individual differences in how time of day influences mood. Anyway, here are a few studies I found on the topic:

Csikszentmihalyi and Hunter (2003) performed an experience sampling study of a large sample of U.S. secondary school students. With regards to time of day they found:


  During the weekdays, time is structured by work or school requirements
  according to a circadian pattern. The ﬁrst part of the day, spent at
  work or school, tends to be less happy, except for a peak at
  lunch-time. There is a dip after lunch, followed by higher reports of
  happiness in the afternoon when one is again free (see Figure 2).


However, day of the week was also relevant with the greatest happiness observed on Saturday.


  Clearly, the social structure of time has an impact on happiness: The
  early part of the weekend, with its freedom from work or school, is
  experienced as liberating. The effect is probably greater on adults,
  for whom the working week is presumably even more constraining than it
  is for teenagers.


Rusting and Larsen (1998) measured negative affect in a small sample of undergraduates where three measure were obtained each day, once each in the morning, afternoon, and evening. The mean levels of negative affect seemed to be slightly higher later as the day progressed. 


  Table 1 Average Percentage of Negative Affect Peaks at Each Time of Day

               M       SD
Morning      30.46   12.10
Afternoon    33.77   13.45
Evening      35.77   13.10

  
  Note. Percentages at each time of day were calculated for each subject
  across the 60-day assessment period. These percentages were then
  averaged across all subjects (N = 46).


Stone et al (1996) measured mood and activity using a diary study with measures taken every 15 minutes for one day. They found:


  Diurnal cycles were found for several moods (e.g., happy, tired),
  activities (e.g., discourse, thinking), and locations (e.g., home,
  work). Diurnal cycles in most moods were strongly associated with
  activities and location, yet diurnal cycles of some moods (rushed,
  sad, and tired) were not dependent on these factors.


If you read their article, you'll see various graphs of self-reported emotions throughout the day both controlling and not controlling for type of activity.

References


Csikszentmihalyi, M. &amp; Hunter, J. (2003). Happiness in everyday life: The uses of experience sampling. Journal of Happiness Studies, 4, 185-199. http://www.mendedreality.com/private/docs/CSIKSZENTMIHALYI2003.pdf
Rusting, C.L. &amp; Larsen, R.J. (1998). Diurnal patterns of unpleasant mood: associations with neuroticism, depression, and anxiety. Journal of Personality, 66, 85-103.
Stone, A.A., Smyth, J.M., Pickering, T. &amp; Schwartz, J. (1996). Daily mood variability: Form of diurnal patterns and determinants of diurnal patterns. Journal of Applied Social Psychology, 26, 1286-1305.