Dietary habits not only impact on physical and mental health; there is now increasing concern about associated impacts on the wider environment, which will subsequently have further impacts on individual wellbeing. We will return to this topic below in section \ref{479675}. We now turn to the role of sleep as a contributor to individual wellbeing.
Sleep
Sleep deficiency is a growing problem brought about through societal changes, including longer working hours and long commutes, shift work, and deteriorating work-life balance due to technology \citep{Luyster2012}. The NHS state that most adults need 6-9 hours of sleep per night \cite{2009}. This seems to be roughly in line with guidelines in the research literature. A recommended sleep statement from the American Academy of Sleep Medicine and Sleep Research Society \cite{Watson2015} had a panel of 15 experts review scientific evidence addressing relationship between sleep duration and health. They concluded that 7 hours or more sleep per night produces optimal health in adults. Sleeping less than 7 hours a night is associated with obesity diabetes, hypertension, heart disease, stroke, depression and increased mortality. Moreover, a recent prospective twin study \cite{Åkerstedt2017} found that both short (defined as <6.5 hours) and long (defined as ≥9.5 hours) were linked to increased mortality. However, it is important to note that these statistics are generalisations, with small groups of people fully able to function healthily on shorter periods of sleep. For example, research highlights that individuals with a mutation in the DEC2 gene required less sleep than their relations without this mutation \citep{Pellegrino2014,He2009}.
Critically, poor sleep is also associated with common mental disorders; many patients suffer from insomnia, while improving sleep in these patients can lead to mental health improvements \citep{Freeman2017}. Analyses on nearly 100,000 adolescents in Japan found a U-shaped association between mental health status and sleep duration \citep{Kaneita2007}. The authors also reported a positive correlation between mental health status and subjective sleep assessment. Similarly, among an elderly population, sleep problems were associated with worsened mental and physical health-related quality of life \citep{Reid2006}.
As with physical activity and diet, changes in sleep are associated with changes in vagal function, such that reduced vagal function (combined baseline and reactivity measures) is associated with sleep disruption \citep*{El-Sheikh2013}. Intriguingly, increases in resting state vagal function have been shown to predict better subjective and objective sleep quality \citep{Werner2015,Grimaldi2016}. By contrast, reduced HRV (as measured over a 24-hour period) has also been detected early during early stages of sleep-related breathing disorders \citep{Aeschbacher2016}. With respect to our GENIAL model \cite{Kemp_2017} and relevance to the following section on community, sleep deficiency and minor, day-to-day reductions in sleep trigger pathways to social withdrawal and loneliness. One pathway through which this occurs is cortical hypersensitivity to human contact, along with impairment in the capacity to recognise social intent \citep*{Ben2018}. Furthermore and relevant to our consideration of wider environmental impacts on individual wellbeing (see section \ref{479675}), nature has been used as an intervention for sleep improvements with promising results. For example, forest walking - involving positive psychological experience (e.g. positive emotions) and health behaviours (e.g. physical activity) - has been demonstrated to improve length and quality of sleep \citep{Morita2011}. It is also interesting to note that individuals who live closer to green spaces are more likely to achieve improved sleep duration (8 hours), even after controlling for well-known sleep influencers, including mental and physical health \citep*{Astell-Burt2013}.