Sleep is an important process in human life that plays a vital role in repair and recovery of body function as well 
“A good night sleep can help eliminating fatigue, restore physical strength, energy and ensure well-functioning of the body and mind.1
Sleeping position or posture of sleep is defined by the alignment and spatial arrangement of spine with respect to neck and extremities.
“Our Habitual sleep position is one of the most important factor that influence the quality and quantity of sleep at night.”
An inadequate position while sleeping could be a cause for nervous tension leading to pain and disrupted sleep system. Sleep is essential to maintain the harmony of musculoskeletal as well as nervous system. A best sleeping position is the one that reduces stress, relaxes muscles, calms mind and facilitates a better body-mind balance.2
This article explores the various sleeping positions, pros and cons of various sleeping positions, recommended sleeping postures for various health conditions and some modifications to improve the comfort and quality of sleep.
Sleep positions – Interesting Facts
Sleep serves as a fundamental biological process for human body and mind. It is a complex action influenced by 
“Many researchers and health professionals have been trying for years to find which is the best and healthiest sleep position. There is no simple answer to this question.”
We begin our sleep in certain body position and usually shift to a more comfortable position either spontaneously or intentionally as the sleep evolves.
“Adults tend to change their sleeping position quite frequently ranging from 11-13 changes per night upto 20-45 times during a typical 8-hour night sleep.3 “
Postural changes are important during sleep to avoid pressure overloading over soft tissues of one area and to prevent muscle stiffness the next morning.
“Most people have a preferred sleeping position that helps them to initiate the sleep and ease into deeper stages of sleep.”
However, it varies with age, gender, change of sleep system (bed, mattress, pillow, comforter), and also with health related problems.4
Types of sleep positions:
Sleeping postures are broadly divided into three basic types, supine position or sleeping on back, lateral or decubitus or side sleeping and prone that is sleeping on stomach.
“Sleep Assessment and Advisory Service (SAAS) have further divided these basic sleep positions into six types: fetus (left or right), log (left or right), yearner (left or right), soldier (up or down), freefaller (up or down) and starfish based on the changing positions of arms, thighs, legs and head.”
Inadequate and uncomfortable sleep could be the cause for multiple health problems.5 Every individual has a particular or habitual sleep posture that helps in easing the sleep onset promoting comfortable and quality sleep at night. It is virtually impossible to define a perfect sleeping position4, however different sleeping postures and their influence on sleep and overall health are discussed below:
Sleeping on back/ Supine position: This is the position in which the head, neck, back and spine are all in a 
Effects on back and spine: Sleeping on back can be helpful in reducing the pain in back as the pressure is uniformly divided with enlarged contact area. Extra soft or extra firm mattress can cause the body to either sink in and becomes uncomfortable on the hard surface. Using an appropriately firm mattress with a pillow underneath the pelvic and lower abdomen area helps in neutralizing the spine, relieving the stress and improving the pain.2,4
Effects on breathing: The role of positional therapy in breathing difficulties in obstructive sleep apnea (difficulty in breathing while sleeping) and snoring are well researched topics. Several studies have reported that the incidence of episodes and severity is totally dependent on the time spent or not spent in supine position. Sleeping on back not only increases the frequency but also increases the length of apnea, degree of oxygen de-saturation and severity of tachy or brachycardia. Sleeping in supine position also increases the risk of airway obstruction by blocking it with tongue during the episodic apnea.6
Effects on eye and brain: Studies have reported an increase in intraocular pressure (IOP) from upright to lying transitions. The changes in IOP are relatively less damaging in back or side sleepers as compared to prone positioning of sleep.7
Effects on digestion: Sleeping in elevated supine positions can help with lowering the frequency of acid reflux episodes at night. However, sleeping in lateral position is considered as best for the patients suffering from gastroesophageal reflux disease.8
Sleeping on stomach/ Prone position: In this position, torso and legs are in one line while lying on stomach 
Effects on neck and spine: In this position with the combined effect of body weight resting on rib cage and intestines pressed against diaphragm, the pressure on lungs increases which can cause further breathing issues. To facilitate breathing, in this position head is turned either to left or right, which increases the stress on neck and subsequently on spine.9 Considering all the disadvantages, mainly increased tension in neck and spine, studies have reported 5-7% of the population prefer to sleep on stomach. It is therefore advisable to improve the support by either sleeping without a pillow under neck to reduce the stress on neck while rotation or by using a pillow under the belly to restore the natural curvature of the spine. This can further be improved by a slight modification by elevating the leg on the side to which head is tilted by using a pillow under the knee to support the spine and reduce long term complications.4
Effects on breathing: Although sleeping on stomach increases the pressure on rib cage and lungs, which in turn promotes the breathing difficulties. However, in individuals with obstructive sleep apnea and snoring, sleeping on stomach might be helpful. Studies have reported that sleeping on stomach/ prone position reduces the chances of airway collapse due to tongue obstruction and helps in improving apnea-hypopnea index (AHI) and oxygen desaturation index (ODI) in comparison to supine position.10
Effects on heart: It has already been discussed that sleeping on stomach increases the stress on rib cage and lungs, which could lead to significant decrease in stroke volume of the heart. This in turn activates the pressure receptors to increase the nerve activity to increase the heart rate and can cause high blood pressure. 10
Effects on ocular pressure: Increased intraocular pressure (IOP) has been identified as a risk factor for increased incidence of glaucoma (loss of partial or total eye sight). Postural change has been associated with the changes in the pressure on eyeball. Studies have suggested that increase in IOP is more commonly associated with prone sleep position as compared to supine or lateral.7
Effects on brain: Individuals with epilepsy, who prefer to sleep in prone position have relatively higher chances of sudden unexpected death syndrome as compared to other sleep positions.
Effects on pregnant women: With the use of appropriate pillows prone position is considered safe for pregnant females. This position is also utilized in massage for relaxation during pregnancy as it helps in improving the blood flow to the uterus. Hypertension is a common condition seen in pregnant females and is a major risk factor for complications and death of infant. Studies have reported that sleeping or lying in prone position with supporting belly pillows help in reducing the blood pressure by reducing the compression on the abdominal vessels.11
Effects on infants: Several studies have documented the relation of sudden infant death syndrome (SIDS) and prone sleep positions in infant. It is also commonly associated with the impairment of arousal from sleep in healthy infants. Various mechanisms such as accidental suffocation, oropharyngeal obstruction, displacement of mandible posteriorly, increased upper respiratory airway resistance, rebreathing of carbon dioxide, overheating, formation of nasal bacterial toxins, impairment of autonomic function, defective arousal response are considered as risk factors which explain the sudden death syndrome in infants with habit of sleeping on stomach.12
Sleeping on Side/lateral/decubitus position: The lateral or side or decubitus is the most adopted sleeping posture. In this position, we are able to support spine correctly as spine column is in straight line with the frontal plane. This position helps in maintaining all the natural curves of the spine. The ideal lateral position for sleep is formed by lying side ways with slightly bending the knee and the hip joint.2 There is not much difference between sleeping on right or left side except for the weight of the liver on the stomach and lungs while sleeping in left decubitus position. Due to decreased contact area and gravitational forces, a side position is an unstable position, which can be balanced by the correct positioning of the extremities, bending arms and legs increasing the contact area and improving stability and sleep.4 Some habitual modifications of basic lateral position include foetal position, the position (left or right), the yearner position (left or right).5
Fetal position: It is one of the most commonly adopted sleep modification of lateral position. In this variation 
Log position: This position is presented by sleeping on side with the arms down next to the body. Sleeping in this position helps in breathing difficulties or snoring.5
Yearner position: This position is presented by sleeping on sides with the arms outstretched in front of the body.5
Effect on back pain: Sleeping on lateral or side gives a more neutral and aligned spine, which helps in reducing the stress in back. Due to the instability of side sleeping because of the less contact area, using an appropriate pillow between the knees and bending arms and legs right in front of body helps in increasing the contact area and stabilization of spine thus releasing the stress off the back.13
Effect on ocular tension: Increase in IOP has been associated with change in position from upright to lying in patients with glaucoma. In habitual lateral sleepers, an increase in IOP has been observed with progression of glaucoma. The continuing habit of side sleeping may also be associated with subsequent loss of vision in patients with glaucoma.7
Effect on digestion: Position of body has shown to highly influence the gastrointestinal reflux before and after meals. Studies have reported that sleeping in lateral or side posture helps in reducing the acid reflux or episodes of gastroesophageal reflux disease relative to spine or prone positions. Further research has documented a more positive outcome while sleeping on left lateral position as compared to right lateral position.8
Effect on breathing: Effect of sleeping postures on breathing difficulties is well documented in literature. Despite the fact that it is difficult to answer the question which is the best sleeping posture, but sleeping on side appears to have more advantages as compared to supine or prone position of sleep. Studies have suggested that positional therapy to learn sleeping on side in obstructive sleep apnea with the use of modern day breathing aids helps to overcome snoring, apneas, hypoapnea and other breathing difficulties.3 Another study, evaluated the effects of right and left lateral sleep positions on the parameters of sleep apnea and discovered sleeping in right lateral position has significant influence on the frequency of apneic events with decreased frequency of obstructive 
Effect on heart: Over the past few years, OSA has become an important risk factor for a range of heart diseases including coronary artery disease, myocardial infarction, hypertension and heart failure. As already discussed, sleeping on side helps in reducing the symptoms of breathing obstruction and improving the quality of sleep.15 Ozeke O, et al. (2011), studied the effect of right and left lateral positions on the outcomes of OSA and heart failure. They observed that patients with heart failure and breathing difficulties find it difficult to sleep on left side in comparison to right side as it interferes with the free action of the heart and increases the nervous activity leading to rise in systemic blood pressure.10
Effect on pregnant females: Sleeping in supine position in late pregnancy is known to compromise the hemodynamics i.e. compresses the major veins resulting in decreased return to the heart, reducing the cardiac output, impairing the uterine perfusion ultimately leading to decrease in maternal blood pressure and subsequent negative impact on fetus. American Heart Association recommends a full left lateral tilt positioning for the women in their late pregnancies with hemodynamic distress.16
Shoulder pain: With all the positive aspects of side sleeping, one notable drawback is increase in risk of shoulder pain. While sleeping on your left or right side, the corresponding shoulder can collapse into the mattress as well as up towards the neck causing a misalignment and pain the next morning.17
FAQ’s
Q1. What is the right way to sleep?
A1. Every individual has their own preferred position of sleep that helps initiate the sleep and ease into deeper stages of sleep. Using a good quality pillow with an optimum mattress in a sleep friendly space is the right way to a comfortable sleep.
Q2. Is it better to sleep on right or left side?
A2. Sleeping on left side increases the strain on stomach and lungs due to the weight of working liver, so its better to sleep on right side. But in individuals with issues of heart burn or acid reflux sleeping on left side is recommended. In pregnant females it is advisable to sleep on left side to maintain the blood flow.
Q3. What is the best position for your spine?
A3. Just remember alignment is the key. Sleeping on back or side or stomach but maintaining the natural alignment of spine is the right way to sleep. if you are side sleeper, placing a pillow between your knees can help neutralize the spine. If you are stomach sleeper, don’t force yourself to other positions, instead of head, place a pillow under your pelvis and lower abdomen to relieve the pressure off the spine. In case of back sleepers, place a pillow under your knees to maintain the natural curve of your spine.
Q4. Is sleeping on left side bad for your heart?
A4. Sleeping on left side after a congestive heart failure can be uncomfortable due to enlarged heart beating against the chest wall.
Q5. Why does putting a pillow between your legs feel good?
A5. Putting a pillow between your legs helps in the neutral alignment of the spine following the natural curvature. It helps in relieving the excessive pressure on the back.
Q6. Where do i put my arms when sleeping on my side?
A6. While sleeping on side, the arms should be rested down by your side. Keeping your arms overhead or under the head on the pillow can lead to muscle soreness and pain in shoulders.
Q7. Is sleeping on your stomach bad?
A7. Sleeping on stomach causes unequal distribution of pressure on spine and back, which could lead to discomfort and poor quality of sleep.
Q8. Is it good to sleep without a pillow?
A8. Sleeping with or without pillow depends on the preferred sleep position. If you are a stomach sleeper, sleeping without a pillow can help reduce the stress on neck and can improve alignment. On the other hand, in case of back or side sleepers, using a comfortable pillow between your knees or under pelvis or lower back can help to attain a better and comfortable sleep.
Q9. Should you sleep with shoulders on the pillow?
A9. It is not advisable to sleep with shoulders on the pillow as it increases the stress between the back and shoulders causing muscle soreness and pain.
Conclusion
Sleep is considered as one of the fundamental process of human body that helps in regaining the strength, relaxes the mind and prepares our body for the following day. A reliable sleeping posture is as important as the other physical and environmental attributes of sleep. it is virtually not possible to obtain an ideal sleep position. As the night progresses, our body has a tendency to change postures with the changing sleep cycles in order to maintain adequate pressure changes and mental activity. In healthy individuals sleeping on back or side are equivalent. However, comfortable sleep on stomach can be achieved with slight modifications of arm, leg and with the use of pillows. To conclude, every individual has a preferred sleeping position, which makes it easy to initiate sleep and provide quality sleep.
References
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- https://dl.uswr.ac.ir/bitstream/Hannan/131758/1/Bart_Haex_Back_and_Bed_Ergonomic_Aspects_of_Sleeping__2004.pdf
- Li Y, Lei Y, Chen LC and Hung Y. Sleep posture classification with multi-stream CNN using vertical distance map. International Workshop on Advanced Image Technology (IWAIT), Chiang Mai, 2018, pp. 1-4, doi: 10.1109/IWAIT.2018.8369761.
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- Kim KN, Jeoung JW, Park KH, Lee DS, Kim DM. Effect of lateral decubitus position on intraocular pressure in glaucoma patients with asymmetric visual field loss. 2013 Apr;120(4):731-5. doi: 10.1016/j.ophtha.2012.09.021.
- Khoury RM, Camacho-Lobato L, Katz PO, Mohiuddin MA, Castell DO. Influence of spontaneous sleep positions on nighttime recumbent reflux in patients with gastroesophageal reflux disease. Am J Gastroenterol. 1999;94(8):2069-73. doi: 10.1111/j.1572-0241.1999.01279.x.
- Cary D, Briffa K, McKenna L. Identifying relationships between sleep posture and non-specific spinal symptoms in adults: A scoping review. BMJ Open. 2019 Jun 28;9(6):e027633. doi: 10.1136/bmjopen-2018-027633.
- Ozeke O, Ertan C, Demir AD. Sleep apnea, heart failure, and sleep position. Sleep Breath. 2012;16(4):933-5. doi: 10.1007/s11325-011-0611-0.
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- Hodkinson JB, Gordon SJ, Crowther RG, Buettner PG. Time to stabilisation of the cervical spine when supported by a pillow in side lying. 2013;56(9):1474-85. doi: 10.1080/00140139.2013.819938.
- Ozeke O, Erturk O, Gungor M, Hızel SB, Aydın D, Celenk MK, Dıncer H, Ilıcın G, Ozgen F, Ozer C. Influence of the right- versus left-sided sleeping position on the apnea-hypopnea index in patients with sleep apnea. Sleep Breath. 2012;16(3):617-20. doi: 10.1007/s11325-011-0547-4.
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