The goal of these series of N-of-1 trials is to compare the effects of three tea interventions on cognition, mood, and sleep, in healthy adult participants. The main questions they aim to answer are: * What are the short-term effects of, and differences between, three different black tea interventions on cognition, mood, and sleep, in individual participants? * What are the short-term effects of, and differences between, three different green tea interventions on cognition, mood, and sleep, in individual participants? * Are there any other lifestyle factors that influence the relationship between tea intake and cognition, mood, or sleep, and to what extent do they have an effect? Participants will be asked to drink three different tea interventions in four blocks of three weeks, where each week is assigned one tea intervention. At regular intervals three times per day, seven days per week, for the 12-week study duration, participants will be asked to complete a sleep questionnaire, mood questionnaire, personalised questionnaire (with questions pertaining to physical activity and work, for example), a tea consumption recall questionnaire, and two cognitive tasks based on attention (lasting one minute each). These questionnaires and tasks comprise one measurement point and take approximately five to six minutes to complete.
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The cognitive domain 'attention' measured using mean reaction time and accuracy in the attention switching task
Timeframe: From enrolment to the end of the final tea intervention. This is the total study duration of 12 weeks.
The cognitive domain 'attention' measured using mean reaction time and accuracy in the digit vigilance task
Timeframe: From enrolment to the end of the final tea intervention. This is the total study duration of 12 weeks.