Most people, when they go to bed, aim to sleep until the morning - but some wake up and are active in the middle of the night.
Last week, we published a story aboutthe myth that an eight-hour sleep is necessary for good health. In response, 10 people told us how they sleep in two separate chunks - and what they do in between.
Take photos of the city
Brennan Wenck-Reilly, 36, San Francisco, US
I spent two years living high in the Andes in a town that had no electricity where I went to sleep and rose with the sun. Now I naturally wake around 01:00 or 02:00 and then become tired again around 03:00 and sleep until 07:00 or so.
My friends have always made fun of my sleep pattern and my wife used to force me to get out of bed as my lying there would disturb her.
I've decided to use this time creatively and run around San Franciscotaking pictures in the night. We just had a small storm pass through the city, so there are some cool clouds in the sky. I have been itching to go out.
Late night yoga
Annelieke Dirks, 33, Haarlem, The Netherlands
I wake up at about 04:00 every night to practise yoga.
Most of the time I do it at home, but once a week I drive to a yoga studio in Amsterdam where I practise with about 20 other people.
I teach and practise kundalini yoga, which has followers all over the world who live in a similar rhythm.
I go back to sleep but some people don't. I find a lot of yogis don't sleep very much.
Sometimes, my partner joins me, if he's having trouble sleeping, but mostly he just sleeps through it.
I used to sleep eight or nine hours a night and still always felt tired. But I've never felt so good in my life as I do now.
Carolyn Cornell, 78, Austin, Texas, US
As an artist, many of my paintings have come directly from my dreams.
I usually wake up around 03:00 and sometimes I have such a strong image in my head that I have to grab a piece of paper and draw it immediately.
I trained as an art therapist and used to work with a young boy called Jonathan who had cancer. He was writing a cartoon story and we were working to put it together when he had a bad relapse and died.
Afterwards I dreamt about him and one night I woke up with a vivid picture of his face. Above it was a speech bubble filled with different colours.
I drew the image immediately and later made this painting of him.
Reham Samir, 25, Cairo, Egypt
As a Muslim, I get up to pray in the middle of the night.
We are advised to sleep and then wake up in the third stage of the night before sunrise, though this night-time prayer is voluntary. It's calledTahajjud, the Arabic word for night prayer.
I usually get up and pray alone rather than going to the mosque. We also have a dawn prayer, which is obligatory, so sometimes I stay up until dawn and then go back to sleep.
According to the Koran, Muhammad slept in this way. So segmented sleep is quite normal in the Muslim tradition.
Play with friends in the tribe
Iain Wilson, 37, Papua, Indonesia
I grew up among the Yali people in Papua, Indonesia. When I was born, my parents lived in a remote area. My mother was a medical worker and my father was an anthropologist. We lived there until I was 16, and as a child I used to camp and go hunting with my friends in the Yali tribe.
When I stayed with them, we would go to bed more or less after sunset and people would always wake up during the night.
I would hear them talking and someone would start a fire. Sometimes we would eat some sweet potato before going back to sleep until 05:30 or 06:00.
At home with my parents, I would get a regular eight-hour sleep, but when I was with my friends, I slept like they did.
Candra Caballero, 36, Daytona Beach, Florida, US
My husband is a sous-chef in a restaurant and gets home around midnight. We eat together, catch up and might watch a quick show on TV, until about 03:00.
Everyone I know tries to discourage me, saying my sleeping habits are unhealthy, but I feel great.
I work a normal eight-hour day and as soon as I get home, my body starts shutting down. I go to bed for about four hours, then get up and start doing chores and preparing dinner.
We've been doing this for about two years. Before then, I used to try to stay up until midnight and get eight hours' sleep, but I used to wake during the night and start to panic.
Watch Korean TV dramas
Karen Rochon, 69, Sequim, Washington, US
Ever since I was a toddler, I have woken up at night. I used to hop into the cot with my brother. It didn't become a problem until I was a student, when I had to get up early to go to classes.
I would try to force myself to get back to sleep but would end up lying there, thinking and getting very frustrated. That went on for eight years until I became a nurse and started working the night shift.
Now I am retired and I get up and watch Korean dramas on TV - I don't speak Korean but they are subtitled. It's quiet and I can watch them for two or three hours undisturbed. I've watched more than 70 series now.
The Korean people have really strong morals. Unlike on American TV, there is very little violence and adultery, the actors are great and they've got great singers too.
Think about dreaming
Bernie O'Leary, 55, Cirencester, UK
Most nights I wake up in the middle of the night and I feel great, full of beans. I could get up and do some chores or walk my dogs but I know it is better to stay in bed. I lie there and look forward to my dreams.
When I was a full-time teacher and had a busy workload in the morning, I used to get anxious that I wouldn't fall back to sleep.
Now I have the luxury of knowing that I have at least two hours of wonderful sleep to come with brilliant dreaming. It is like having your very own home cinema to look forward to every night: the sheer expectation sends me to sleep again.
My dreams always reflect what I was thinking that day. Through them, I reconcile issues from the day and solve problems.
Meditate at church
Theresa Laturnus, 58, Surrey, British Columbia, Canada
I haven't been able to sleep through the night since I was pregnant with my first child, 24 years ago. Also, my husband snores like a Harley Davidson motorcycle, so I used to go sleep on the sofa in the living room.
Recently he has been having trouble sleeping and wakes in the night too. We have decided to go to bed early on a Saturday night and get up around 02:30 to go meditate at our church.
It is a Catholic church where they have a chapel that is open all night for special prayers called 24-hour adoration. According to these prayers, someone has to be there 24 hours a day so we relieve the person who lets us in and we stay until someone else shows up.
That hour usually flies by. We come home relaxed, go back to bed and sleep again until the morning.
Listen to music
Jiri Janata, 72 Atlanta, Georgia, US
I didn't always wake up at night; it is something that developed with age. At first I would stay in bed and worry, but now I listen to music to pass the time.
I sneak out of my room, so as not to wake my wife, and go to my study where I turn on theNPRprogramme Music Through the Night.
After that, I go back to sleep for another two hours. It is wonderful - the anxieties that I used to experience when staying awake in bed worrying are gone. Music is the answer.