The investigators hope to learn if low sodium oxybate (LXB) is an effective treatment for symptoms of idiopathic hypersomnia (IH) and postural tachycardia syndrome (POTS). Previous research has shown that patients with IH also report having symptoms associated with POTS. The researchers have observed that in patients with both IH and POTS, when patients' sleep quality improves, so do their POTS symptoms. The goal of this study is to test this in a controlled way by using LXB as a treatment for both IH and POTS in patients that have been diagnosed with both conditions.

Inclusion Criteria:

1. Age ≥ 18 years
2. English speaking and capable of signing the informed consent form and comply with the protocol requirements
3. Diagnosis of IH based on ICSD-3 criteria
4. Diagnosis of POTS based on the 2011 Consensus Criteria
5. Oxybate naïve. Participants on daytime alerting medications at baseline can remain on these medications for the duration of the study, provided doses remain stable for the duration of the study. This rule also applies to medications prescribed for the treatment of POTS. All medications known to affect cardiovascular function will be held for the appropriate time periods prior to active stand testing so as not to influence results.
6. COMPASS 31 ≥ 40 at the screening visit
7. Agree to use contraceptives consistent with local regulations regarding the methods of contraception for those participating in clinical trials

Exclusion Criteria:

1. Hypersomnia due to another medical, behavioral, sleep, or psychiatric disorder
2. Evidence of untreated or inadequately treated sleep disordered breathing, defined as AHI ≥ 10
3. History of bipolar or psychotic disorders, major depressive episode, or current suicidal risk
4. Pregnant or lactating or intends to become pregnant during the study
5. History of or currently being treated for clinically significant ongoing cardiac arrythmia, heart failure, myocarditis, pulmonary embolism requiring anticoagulation, pulmonary fibrosis or other pulmonary diagnosis that in the investigator's opinion may contribute to symptoms of POTS
6. Clinically significant uncontrolled active or chronic bacterial, viral, or fungal infection or positive SARS-CoV-2 PCR test at screening
7. A medical condition that could confound the results of the study or put the participant at undue risk in the investigator's judgment
8. Intends to have surgery or procedures requiring anesthesia or conscious sedation during the study
9. Currently participating in another interventional clinical study
10. Unwilling to remain on a stable regimen of medications during the study
11. Use of or planned use of intravenous saline infusions during the study