Changes

Boundaries of days are another common issue when it comes to handling dates & times for personal science. While our calendar and clock progress to the next day every midnight, for many people this does not accurately reflect how they think of a ''single day''. Instead, a ''day'' is experienced as the time period between wake-up and going to sleep again. Despite this, many tools and in particular wearable devices (such as Apple Watch and Fitbits) enforce a day break at midnight, meaning that e.g. activity, heart rate and other variables recorded after midnight (but before going to sleep) is already recorded for the new day.  

Boundaries of days are another common issue when it comes to handling dates & times for personal science. While our calendar and clock progress to the next day every midnight, for many people this does not accurately reflect how they think of a ''single day''. Instead, a ''day'' is experienced as the time period between wake-up and going to sleep again. Despite this, many tools and in particular wearable devices (such as Apple Watch and Fitbits) enforce a day break at midnight, meaning that e.g. activity, heart rate and other variables recorded after midnight (but before going to sleep) is already recorded for the new day.  

At the same time not all devices default to the calendar-based standard: E.g. the Oura Ring devices provide daily aggregate meetings which measure a ''day'' as the time period between sleep periods. This means one needs to be cautious when comparing daily summary statistics from different sources, as they might use different ways to define a ''day''.

At the same time not all devices default to the calendar-based standard: E.g. the [[Oura Ring]] devices provide daily aggregate meetings which measure a ''day'' as the time period between sleep periods. This means one needs to be cautious when comparing daily summary statistics from different sources, as they might use different ways to define a ''day''.

=== Which day is it? ===

=== Which day is it? ===

Not only can are the boundaries of days problematic: For activities which typically span across day-boundaries, one might take different approaches regarding to which day the data should be associated. Tracking sleep is a typical example of this: If you go to sleep before midnight on day X and wake up on day X+1, for which date do you want to associate the summary statistics of your night's sleep?  

Not only can are the boundaries of days problematic: For activities which typically span across day-boundaries, one might take different approaches regarding to which day the data should be associated. Tracking sleep is a typical example of this: If you go to sleep before midnight on day X and wake up on day X+1, for which date do you want to associate the summary statistics of your night's sleep?  

Some tools (e.g. Fitbit) will choose day X+1 as the day to associate the sleep, i.e. the day on which you wake up. Other tools (e.g. the Oura Ring) will choose day X as the day for the sleep summary statistics, i.e. the day you went to sleep (Interestingly this even happens if you go to sleep ''after'' midnight).  

Some tools (e.g. [[Fitbit]]) will choose day X+1 as the day to associate the sleep, i.e. the day on which you wake up. Other tools (e.g. the [[Oura Ring]]) will choose day X as the day for the sleep summary statistics, i.e. the day you went to sleep (Interestingly this even happens if you go to sleep ''after'' midnight).  

It can be important to understand this distinction both when comparing different sleep metrics to each other, in order to compare data from the correct dates but also when trying to understand how sleep might affect other variables. Not understanding for which day the sleep is recorded can easily result in being off-by-one.

It can be important to understand this distinction both when comparing different sleep metrics to each other, in order to compare data from the correct dates but also when trying to understand how sleep might affect other variables. Not understanding for which day the sleep is recorded can easily result in being off-by-one.