--> This is a substudy of the main ESTxENDS trial (NCT03589989). Sleep quality outcomes should be considered secondary outcomes of the main smoking cessation outcome formulated in NCT03589989.

Cigarette smoking is the leading cause of preventable death in Switzerland and still more than a quarter of the Swiss population smokes cigarettes. Recently, electronic nicotine delivery systems (ENDS; also called vaporizer or electronic cigarette) have become popular with smokers who want to stop smoking or reduce their exposure to inhaled chemicals since ENDS use appears to be safer than tobacco smoking.

The majority of attempts to quit tobacco smoking do not end in success due to unpleasant withdrawal symptoms. One of such symptoms is disturbed sleep. Sleep disturbances are a problem for smokers who are trying to quit. Sleep disturbances results from difficulty falling asleep or frequent arousals after sleep onset, and have negative daytime consequences such as sleepiness and dysphoric mood. Studies indicate that awakenings during night affect the cardiovascular system by providing repetitive bursts of sympathetic nervous system activation, likely contributing to elevated levels of cardiovascular and cerebrovascular morbidity. ENDS with nicotine containing e-liquids may be effective in assisting with tobacco smoking cessation by reducing withdrawal-induced sleep disturbances.

This study will therefore test the efficacy of ENDS for cigarette smoking cessation, the safety of ENDS on adverse events and the effect of ENDS on health-related outcomes and exposure to inhaled chemicals.

For the main ESTxENDS trial (NCT03589989), cigarette smokers motivated to quit smoking cigarettes will be included. Participants in the intervention group will receive an ENDS and nicotine-containing e-liquids, which they will be allowed to use ad libitum. Additionally, they will receive smoking cessation counseling. Participants in the control group will receive smoking cessation counseling only. All participants will be followed over a 24-month period. Sleep quality will be assessed using the Pittsburgh sleep quality index- questionnaire at baseline and at 6, 12 and 24 months' follow-up.

Inclusion criteria:

• Informed Consent as documented by signature
• Persons aged 18 or older
• Currently smoking 5 or more cigarettes a day for at least 12 months
• Willing to try to quit smoking within the next 3 months,
• Persons providing a valid phone number, a valid email address and/or a valid postal address.

Exclusion criteria:

• Known hypersensitivity or allergy to contents of the e-liquid
• Participation in another study with investigational drug within the 30 days preceding the baseline visit and during the present study where interactions are to be expected
• Women who are pregnant or breast feeding
• Intention to become pregnant during the course of the scheduled study intervention, i.e. within the first 6-months of the study
• Persons having used ENDS or tobacco heating systems regularly in the 3 months preceding the baseline visit
• Persons having used nicotine replacement therapy (NRT) or other medications with demonstrated efficacy as an aid for smoking cessation such as varenicline or bupropion within the 3 months preceding the baseline visit
• Persons who cannot attend the 6- month follow-up visit for any reason
• Cannot understand instructions delivered in person or by phone, or otherwise unable to participate in study procedures

Statisticians and laboratory personnel will be blinded to group allocation