Guide · Daylight & housing
Daylight deficit: why so many homes miss real sunlight
Studies find that we spend around 90 % of our time indoors. How much real sunlight reaches us there is decided by where the window faces, how high the building opposite is and which floor we live on, not by us. Here is how much sun a home should get, why so many don't, and why winter makes it worse.
How much sun should a home get?
The European standard EN 17037 (Daylight in buildings) recommends that a living space receives at least 1.5 hours of direct sunlight on a day between 1 February and 21 March. In Germany, the earlier edition of DIN 5034-1 (2011) asked for more: at least one living room should get 4 hours of possible sunshine at the equinox and at least 1 hour on 17 January.
These are recommendations, not laws. The current edition of DIN 5034-1 no longer contains the sunlight requirements, and many homes were built long before such standards existed. That is why so many rooms never meet them.
Who is affected
- North-facing rooms get direct sun only briefly on summer mornings and evenings, and practically none in winter.
- Ground floors and lower floors sit in the shadow of the building opposite for months.
- Courtyard rooms and rooms on narrow streets see only a slice of sky.
- Rooms behind balconies or with deep floor plans lose the sun before it reaches the back of the room.
Why winter makes it worse
In Berlin the sun stands only 14° above the horizon at noon on 21 December, and the day lasts about 7.7 hours. In June it climbs to 61°, and the day lasts almost 17 hours. Low sun is easily blocked: if the building opposite, to the south, is 18 m high and 30 m away, a ground-floor window sees no direct sun from mid-October to late February, not even at noon. That is more than four months.
Our own calculation for Berlin (52.5° N), flat ground and a building directly to the south. The free Sun Check estimates the hours for your own window.
Why daylight matters: what research says
Light is the strongest signal for our internal clock. After a week of camping with nothing but natural light, the participants' internal clocks ran earlier and lined up with sunrise and sunset (Wright et al. 2013). An international expert group recommends at least 250 lux melanopic EDI at eye level indoors during the day (Brown et al. 2022). Outdoors, even an overcast sky delivers many times that.
The largest study so far comes from the UK Biobank: around 89,000 people wore a light sensor on the wrist for a week and were followed for about eight years. More daylight during the day went along with a lower risk of death, more light at night with a higher one (Windred et al. 2024). These are associations, not proof of cause, and they are about everyday light exposure in general, including time spent outdoors, not about any product. Our guide Natural light and wellbeing at home looks at more of the research.
What can you do?
Bigger windows, light shelves or a tubular skylight all need building work, and none of them changes the path of the sun. A heliostat takes the other route: it stands where the sun still reaches, outside on a balcony or terrace, and redirects real sunlight through the window you already have. See all options compared.
A note on health: ohoya is a home daylight product, not a medical device. We make no health or therapy claims. Window glass blocks most UVB, so sunlight through a window does not provide vitamin D.
Frequently asked questions
What is a daylight deficit?
A home has a daylight deficit when its rooms get too little direct sunlight because of the window direction, the floor, the buildings opposite or the season. Standards such as EN 17037 give a reference point: at least 1.5 hours of direct sun on a day between February and March.
How many hours of sun does my window get?
It depends on the direction, your latitude and what stands in front of it. The free Sun Check estimates the hours for your window and how many a heliostat could add.
Does sunlight through a window give me vitamin D?
Practically no. Normal window glass blocks most of the UVB radiation the skin needs to produce vitamin D. Sunlight redirected through a window is real daylight for the room, not a source of vitamin D.
Is ohoya a medical or light therapy device?
No. ohoya redirects real sunlight into a room. It is not a medical device and not a light therapy lamp, and we make no health claims.
What does daylight have to do with longevity?
Large observational studies link bright days and dark nights with a more stable circadian rhythm and a lower risk of death (Windred et al. 2024). That is an association across a population, not a promise for any individual, and no study has examined a heliostat.
Real sunlight for the rooms that miss it
The ohoya Heliostat redirects real daylight through the window you already have. The reservation is refundable and is not a purchase.
Not sure how much sun your window gets? The free Sun Check works it out for your location. Or join the waitlist for every milestone update.
This device has not been authorized as required by the rules of the Federal Communications Commission. This device is not, and may not be, offered for sale or lease, or sold or leased, until authorization is obtained.
Sources: Klepeis et al. (2001), The National Human Activity Pattern Survey, J. Expo. Anal. Environ. Epidemiol. 11: 231–252 (about 87 % of time indoors plus about 5 % in vehicles, US data). EN 17037 Daylight in buildings. DIN 5034-1:2011-07. Wright et al. (2013), Entrainment of the human circadian clock to the natural light-dark cycle, Current Biology 23: 1554–1558. Brown et al. (2022), Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults, PLOS Biology 20(3): e3001571. Windred et al. (2024), Brighter nights and darker days predict higher mortality risk, PNAS, doi:10.1073/pnas.2405924121. Sun angles and day lengths: own calculation.