Quantifying the Effects of Daily Tea Consumption on Cognition, Sleep, and Mood, and Understanding Individual Characteristics That Affect Responses: a Series of N-of-1 Studies

Two independent series of 12-week N-of-1 studies will be conducted to separately assess the effects of two primary interventions, black tea flavoured tea drink (containing bioactive compounds equivalent to naturally occurring levels in black tea), and green tea flavoured tea drink (containing bioactive compounds equivalent to naturally occurring levels in green tea), against two controls each (caffeine only, and placebo, flavoured according to the primary interventions), on cognition, mood, and sleep.

For each primary intervention, the 12-week study duration will comprise of four, three-week cycles and each cycle will comprise of three, one-week intervention periods. The order of the three interventions will be determined separately for each participant by randomly selecting four of the six possible combinations of three interventions, and such that no intervention is given in consecutive weeks. Each one-week intervention period will be split into six test days and one washout day. The washout day will be a minimum of 24 hours (00:00 to 23:59). On each of the test days, participants will be asked to drink the tea intervention allocated to that week, as often as they would like. Participants will be asked to refrain from drinking any other tea products, and caffeinated, or decaffeinated, drinks. On washout days participants will also be asked to refrain from drinking the tea products provided as part of the study.

The randomisation process to determine intervention sequences will be conducted by a third party who will not be involved in running of the study or data analysis. Each tea intervention will be coded prior to being delivered to the Rowett Institute and the file linking each tea code to the tea will be sent from Lipton Teas and Infusions to the Quality Assurance Manager for the Rowett Institute. The principal and sub-investigators will be informed which three tea codes relate to the three green tea and three black tea interventions for the purposes of running the study. The specific tea interventions assigned to each code will only be revealed to the principal and sub-investigators once the data analysis for all participants has been completed.

At three time points on each study day, participants will be prompted to complete a series of questionnaires and cognitive tasks which will form one measurement point. These time points will be decided on a per participant basis and where possible one measurement point will fall within each of the following time periods: 0800-1000, 1200-1400, 1600-1800. For participants who work night shifts these time periods will not apply, however, the time intervals between each measurement point will be maintained where possible. The cognitive tasks will be the digit vigilance task and attention switching task, and questionnaires include a sleep questionnaire, mood questionnaire, personalised questionnaire, and tea recall questionnaire. The investigators will use Gorilla Experiment Builder to deliver the questionnaires and cognitive tasks. The sleep questionnaire will only be included in the first measurement point of each day. The investigators will also ask participants to note accidental consumption of any tea or caffeine products not related to the study, but participants will not excluded from the study if they have consumed a product unrelated to the study.

Once potential participants have been screened against the inclusion and exclusion criteria and informed consent has been obtained, participants will complete a series of questionnaires that will be used to identify variables that are likely to show some degree of personal variability over time and may influence their cognition/attention, mood, sleep, and/or compliance to the intervention. Four validated questionnaires will be used: the UK Short-form 12 health questionnaire, the short-form International Physical Activity Questionnaire, the Pittsburgh Sleep Quality Index, and the trait anxiety arm of the State Trait Anxiety Inventory in order to provide an overview of their typical lifestyle-related behaviours. Participants will also be asked to complete a questionnaire detailing their tea consumption and consumption of other caffeinated food and drinks and a questionnaire to establish other lifestyle factors e.g., work routine, dependents, whether the participant lives with another person. The tea consumption questionnaire will also inform whether the participant will be included in the black tea or green tea study. Assessment of these time-dependent variables during the study is known as ecological momentary assessment and will be implemented using semi-personalised short questionnaires delivered via Gorilla Experiment Builder at each of the three daily measurement points over the course of the study.