The so-called “weekend effect” is a well-known phenomenon in the stock market (Keim & Stambaugh, 1984). This relates to significantly lower stock returns on Monday in comparison with other days of the week. It is suggested that this effect could be a result of traders’ fading optimism between Friday and Monday. Interestingly, similar “weekend effects” can also be found in health-related matters. The number of suicides, for example, is higher on Monday in comparison with other days (McCleary, Chew, Hellsten, & Flynn-Bransford, 1991). In addition, calls to the AIDS-STI telephone hotline, an information helpline that people call mainly when they are concerned about their personal risk on having contracted a sexually transmitted infection (STI) (Mevissen et al., 2011a), also peak on Monday (M. Mientjes, STI AIDS Netherlands, personal communication).

How can these peaks in sexual concerns on Mondays be explained? The first explanation that comes to mind is that the frequency of (unprotected) sex is higher on Fridays and Saturdays (Schroder, Johnson, & Wiebe, 2007), resulting in more concerns on what may have gone wrong afterwards (e.g., contracting an STI). Telephone information helplines being closed during the weekend, resulting in an accumulation of concerns, can furthermore explain the high number of calls on Mondays.

Nowadays, however, the Internet is used as well to deliver counselling services (e.g., websites aimed at communicating personal risk on contracting an STI) (Mevissen, Ruiter, Meertens, Zimbile, & Schaalma, 2011b). These services are accessible 24/7. Thus, instead of waiting to be able to call on Monday, Internet-delivered counselling services can be used immediately. Hence, one would expect that the use of these services peaks on Saturday and Sunday, after having had unprotected sex on Friday or Saturday night.

An advantage of Internet-delivered counselling services is that visitors’ behavior (e.g., use of the service) can be tracked by means of server registrations. In contrast to self-reports, tracking visitors’ behavior is independent of visitors’ memory, interpretation or social desirability (Crutzen & Göritz, 2010) and this provides valuable information about the actual use of Internet-delivered counselling services (Crutzen, Peters, Dias Portugal, Fisser, & Grolleman, 2011).

Figure 1 provides an overview of the number of visitors per day during 2 months (April 1–June 1, 2011) for a website of STI AIDS Netherlands (www.soatest.nl) that provides visitors tailored information about the need to conduct an STI test taking into account their sexual behavior. A remarkable finding—given that the website is accessible 24/7—is that the number of visitors per day still peaks each Monday (the only exception being April 25, which is Easter Monday and a Dutch holiday, where the peak is on Tuesday). These figures are comparable to a previous study concerning an emergency contraception website (Wynn & Trussell, 2005) that found peaking e-mails on Monday. The Dutch higher online use rates on Monday cannot be explained by lack of Internet access at home, thus resulting in accessing the website at work, as suggested by Wynn and Trussell (2005), because the Netherlands has one of the highest Internet penetration rates in the world (88.6%, http://www.internetworldstats.com/).

Fig. 1
figure 1

Number of visitors per day (April 1–June 1, 2011; vertical lines indicate Mondays)

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An explanation for the delay between being concerned about contracting an STI (i.e., “the morning after”) and actually taking action by visiting an Internet-delivered counselling service may be the “deliberation-without-attention effect”: deliberation in the absence of conscious attention directed to the problem (e.g., being concerned about contracting an STI). In the vernacular, this is called “sleeping on it.” Previous research demonstrated that choices in complex matters benefit from leaving them to unconscious thought (as opposed to simple choices, such as choosing one’s favorite between different towels) (Dijksterhuis, Bos, Nordgren, & Van Baaren, 2006). So, although Internet-delivered counselling services are accessible 24/7, this does not mean that people actually visit them immediately when they are concerned. This is not necessarily a bad thing from a decision-making point of view; in the end, the services were used. However, since quickening action is called for in the case of a potential STI infection or an unplanned pregnancy, it is important for future research to focus on reasons behind this phenomenon. Furthermore, from a practical point of view, it is recommendable for offline counselling services (e.g., telephone information helplines) to take this weekend effect into account by increasing their capacity on Mondays.